In case you or anyone else still wants the answer to this question. Find the installation directory of the Character Generator (Defautls to Program Files/Character Generator 2.0), from within there is another folder titled "Character Pieces", in there are all character pieces that the Character Generator uses. Simply add the character pieces from Modern Exteriors to the correct folder.
Note in order for a character piece to be loaded it **must** also have a 16x16 verison. So if you want to use the 32x32 or 48x48 versions you need to also include the 16x16 version for it to appear in the Character Generator.