I had assumed you wanted to set up vanilla medium armour as light armour. What you want to do is basically the same process; but there are a few extra steps involved.
As you have discovered, game assets are packaged in ERFs. I assume then that you have actually installed the mod and are browsing through the Addins folder? In that case, to do what you want you just need to extract the appropriate GDAs. You can leave the models and textures untouched.
Using either the toolset or the standalone ErfEditor.exe, open \\Addins\\XT_Phoenix_Armory\\core\\data\\XT_PhoenixArmory_2DA.erf, select armor_light_variation_ph.gda, boots_light_variation_ph.gda, and gloves_light_variation_ph.gda, then right click and Extract Resource. Save them somewhere easy to find. Now open the GDAs in GDApp and select File->Export To->Excel File for each. Now you can open the XLS files in Excel. If you are using 2007/2010, you’ll likely get a couple of error messages pop up. Just click OK to clear them then save the file, overwriting when prompted. That will fix the issue. Do the same for the medium armour GDAs.
Now it’s a simple case of copying the 2 or 3 data rows from each medium armour sheet and pasting them below the data row in the corresponding light armour sheet. Then change the ID numbers of the pasted rows so that each is unique (since all sheets start at 46 and the light armour sheets only have one row, just increase all medium armour IDs by 1). The last thing to do is to edit the sheet names to remove the “.xls” or part thereof. You want to keep the sheet names the same as the original GDA filenames (without the file extension). Once that is done you should have something like this:

You can now delete the extracted medium armour GDA and XLS files and the light armour GDA files, as they are no longer needed. Save the light armour XLS files, then close Excel. Copy the XLS files to <DA install dir>\\tools\\ResourceBuild\\Processors\\ and then drag and drop each file (one at a time) over ExcelProcessor.exe. You’ll see a commandline window flash up briefly and then a GDA should appear in the same directory. If you get an error, it’s likely in the sheet names, so double check those and try again. Once that is done you can delete the XLS files.
Now to create the items. Take the GDA files you just created and paste them into My Documents\\BioWare\\Dragon Age\\packages\\core\\override\\. Also copy across XT_PhoenixArmory_MH.erf, XT_PhoenixArmory_MO.erf, XT_PhoenixArmory_GUI.erf and \\Addins\\XT_Phoenix_Armory\\core\\textures\\high\\XT_PhoenixArmory_Tex.erf as well. The MO/Tex ERFS aren’t strictly necessary, but you won’t get any textures showing up in the toolset without them, so they are useful for visual reference.
In the toolset, create a new module and create your new items. This is covered elsewhere so I won’t go into great detail. You can use whatever tag/resref you want for your items, but I just followed on from the ones in the Phoenix mod. The light armour chest pieces there were phoenixh_armor_lgt.uti and phoenixh_armor_lgt2.uti, so I created the test items phoenixh_armor_lgt3, phoenixh_armor_lgt4, and phoenixh_armor_lgt5. In the item properties, first scroll down to the bottom to the appearance section and set the gender to female (or the models won’t show up). Then back at the top change the base item type to “Armor – Light” and then in the variation drop down choose one of the Phoenix options. You should have 3 variations available – the original light armour model and the two medium armour models you added:

N.B. For the purposes of this demonstration I’m just covering the chest armour, but you’ll obviously want to create boots and gloves as well.
One of the ERFs you copied across included custom icons, which you can use for your new items. They all begin with the prefix “ico_pnxhrt_”, so just paste that into the filename field when browsing for an icon to narrow the search. Once you have the items made to your satisfaction, save them then export them.
Now we need a script to get the items in the game. There are a number of ways to do this, including adding items to merchants, but here I’ll just show you a quick and dirty script that adds the items to the player’s inventory.
First, create a new plot. I called mine “phoenixh_med2lgt”. In the plot editor, right click and choose Insert->Main Flag. Click in the flag name to edit it. I changed mine to “MED2LGT_CHECK_FLAG”. Save the plot then export it.
Create a new script. I called mine “phoenixh_med2lgt”. In the script editor, paste the following:
#include "utility_h"
#include "plt_phoenixh_med2lgt"
void main()
{
if ( WR_GetPlotFlag( PLT_PHOENIXH_MED2LGT, MED2LGT_CHECK_FLAG ) == TRUE )
return;
event ev = GetCurrentEvent();
int nEventType = GetEventType(ev);
switch ( nEventType )
{
case EVENT_TYPE_MODULE_LOAD:
{
UT_AddItemToInventory(R"phoenixh_armor_lgt3.uti", 1);
UT_AddItemToInventory(R"phoenixh_armor_lgt4.uti", 1);
UT_AddItemToInventory(R"phoenixh_armor_lgt5.uti", 1);
WR_SetPlotFlag( PLT_PHOENIXH_MED2LGT, MED2LGT_CHECK_FLAG, TRUE );
break;
}
default:
break;
}
}You’ll obviously want to edit any values that are different on your end, and add additional lines for any extra items you made. Save the script and check the log window to make sure there were no compiling errors. Export the script. What this script does is run every time you load the game. It checks the save for the flag in the plot we made – if the value is false, it adds the listed items to the player’s inventory. If the value is true, it stops. This results in the items only being added once.
Go to File->Manage Modules, highlight your module and click on the Properties button. Click in the Script section and load the script you just made. Go to Tools->Export->Generate Module XML and Generate Manifest XML. You can now close the toolset.
The final step is some cleanup. Go to My Documents\\BioWare\\Dragon Age\\Addins\\XT_Phoenix_Armory\\core\\ and create a new folder. Call it “override” (without the quotes). Move the GDAs from My Documents\\BioWare\\Dragon Age\\packages\\core\\override\\ to this new folder. You can delete the ERFs you copied there (although make sure they are only copies – don’t delete the originals in \\Addins\\XT_Phoenix_Armory\\core\\data\\).
Now you should be ready to test in-game. Just make sure that your module is enabled in the downloadable content list.