You can mod levels to a very limited extend with ME3Explorer's tools. You can import new meshes etc. There seems to be a memory constraint though, so if you add too much, the game will crash. Also, the tools are working but they do have a bit of a learning curve. It's not like using UDK and if you want to add stuff like triggers, etc., that will still require quite a bit of challenging HEX editing.
As for adding new levels, this works now, that we can add new pccs to the game via DLCs.
Adding the new maps as missions may be more difficult and require proper adding of the level to the coalesced in a number of places. AFAIK, it has not been done yet but my own tests from a while ago suggest it should be possible now.
Adding new gameplay has not been done yet to my knowledge. AFAIK, it is not really well researched how enemies are exactly handled.
In conclusion: It is theoretically possible to clone a map from ME3, modify it extensively (to the point where you have a new level) and re-insert it into the game. However, the time and effort for this at the moment would be enormous and -since a lot of it has not been fully researched - you may run into a lot of read block on the way.