I've been trying to figure out the same thing for a while now. All I know is that after you dump the game with something like BrainDump, and then extract the romfs file, you can view and extract the textures and models with ohana3ds rebirth.
So far though, I have never been able to import anything back to the files. The best that I've been able to do is to swap models around by renaming them in \romfsdir\model\body\.
If you can figure it out, be sure to post about it. I'd love to know how to make it work.
Most people only tell you how to mod Pokemon or Animal Crossing, and the methods used for those games don't seem to work for Tomodachi Life.