Well there's to methods.
1. in xnalara export in model as a obj after, moving the mouth to be opened . So rigging the mouth wont be a pain later on. Import the model in blender. Export as a light wav obj. Flip all the textures upside down and save them. Otherwise, mmd will read them wrong. In any pmx editor import the light wav obj models.
2. In blender if you have the xnalara and mmd scripts installed. Import the model, and export as a pmx model. This should have the bones intact.