Is there a way to convert meshes to solids?

I have an old drawing that I made about 15 yrs ago with DesignCAD. I opened it in CorelCAD and see that it is entirely made of PolyFaceMesh segments.

It seems there is no way to convert these to solids.  I thought the 'Thicken' command would do it but it sure doesn't (I actually can't find ANY kind of entity that the Thicken command WILL work on).

Is there any way i could make these into a solid?

Thanks, John