With MMD models you mean, like the game's dresses and such?
Usually, the workflow of converting game content from other games, is to simply see if someone already made a XNALARA (also known as XNA) version of it (Those are usually available on deviantart). Then, just simply export it as a obj and tweak the heck out of it in blender
I'm not too sure how much experience you have with meshing, but if you need some help i'm right here!
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