There really is NO way to merge the clothing meshes to a figure, so it's all one closed "watertight" mesh. The STL plugin is worthless for that application.

You would have to go in from a base obj level in a object editor, and weld all the points from the clothing to the figure, and loft all the flat polys left into full 3d version of them with thickness. For a skilled modeler (myself) I'd say I'd be looking at 2 weeks minimum full time work to make sure the entire thing had become a watertight object.

OOO yeah I just thought of that.. you need to provide the mesh, and reistribution of meshes is illegal. One of the things they mention in the requirements is that you must have redistribution rights to your submission.

So forget all the bought characters and clothing, and accessories.

For all we know, they might outright reject submitted objects based on poser content outright.
Or they might just turn thier heads and look th eother way.. but as soon as you post an image of someting done with copyrighted poser content, everyone will jump down your throat.

Way too many people take way too many things way too seriously.