Not exactly sure what you’re trying to do within Mudbox, but in my experiences Daz integrates horribly with it - I love Mudbox for texture painting (often over Genesis), but they just don’t play nicely together.
Like I said, I mostly use Mudbox to paint textures. When I use Genesis from Daz, he gets a bit warped like you say (using .fbx format)... pretty sure it’s due to something with the skeleton/skinning, but anyway you’re best off to pose Genesis while still in Daz and then export to .obj and import that into Mudbox. That way, there’s no skeleton and no warping.
Hope this helps a little bit.