If I am understanding your question correctly...
Each appearance requires its own MDB model file with associated textures, so to add an appearance you would need to build a new model (or reskin an old one). Doing that right takes time and practice. (You can also go looking to see if anybody else has built other portal models for NWN2.) Once your model is done, you then need to modify your placeables.2da file and add a hak file containing the new model and textures.
Have you checked to see if the portal is tintable? You might be able to change that. You can also change the scale. Another option is to creatively group placeables together so as to produce a different appearance. Different visual effects can also help.