jmmiller,

Fabrication parts converted or otherwise are a series of faces. I am relatively certain no matter how many different ways you approach this you will not get clean 3D solids. Sorry, but I just thought I'd let you know that you are probably wasting your time.

The only possible way to do this will involve extensive and extreme analysis of the faces programmatically to recreate them as solids. I don't know why your doing this, but if this is necessary then you may want to look at a different software. I am not a huge fan of Plant 3D, but I am relatively certain the geometry is based on solids and would be a lot better direction for what your trying to accomplish.