For Poser models and other models that have clothing:

 

your models need thickness so the easiest way to do that would be -

 

Look at the mesh and find points you can close off. Where you need to, you can also turn an item or object into a shell (that's what silo calls giving a model thickness).

Good points to close off - the pants at the opening of both legs and the waist. You can cap those and then delete the geometry of the body that intersects, capping the bits left over and moving them so that they only interesect a little. Some intersection is needed or they'll be milled as separate parts.

ZBrush is good for that part. There's a feature in it that will close open holes in geometry.  There's also a Curve Slice tool, which will make very nice cuts in a model so you can then cap them. I'd illustrate but it would take awhile.