That's actually what I'm working on. As I said before though, the problem that I'm running into is that the bigger I make the belly the more pointy the polygons become(I'm using Blender's inflate tool). I'm going to try and subdivide the belly, and test it out to make sure theres no performance loss. But right now the best I can do is make it look like Serleena from MIB2 which probably isn't all that realistic with the weight.
Edit: Unfortunately subdividing PDX meshes just isn't possible without the ingame model flipping out. Best I can try and do is manually move some of the vertices around to give it a smoother look. I'll post the file in a couple days if anyone wants to play around with making it look better in Blender.
Edit 2: Heres some minor improvements. As you can see theres still some stretching with the textures. In order to fix it I'd probably have to edit the UV map, which is something I don't know how to do. But for now we can just pretend they are stretch marks. I wanted to make the belly button poke out more, but since I need the texture in order to find it, I had to guess, which I don't think I did a very good job with. I'll try and fix that next.
Update: I figured out how to load textures, and match the UV wrap. So expect to see an even smoother belly from me soon.