You will find all the textures located in
"C:\Program Files\The Game Creators\FPS Creator X10\Files\entitybank\"
Then inside there respectively will be your scifi/ww2 etc.
You will see a directoy for characters and inside them will most likely be dds files (example: ai_1_D.dds). You will need a plugin (if you are using photoshop) to view these files. I believe Gimp also works with them.
Anyway edit the files you want. _D is the main texture, then you have normal maps _N and specular maps _S.
the _# (_1, _2) is just the texture array for that character. So technically could add a new one for Aiko and just name it: aiko_6_D/S/N.dds