If i'm not wrong, Autodesk Inventor is based around ACIS core. ACIS-based software always were very bad in terms of handling and operating meshes like STL. The only way I see is to reduce the point count of mesh using something (...Blender... free and fast enough) and use this low-poly mesh as a dummy in Inventor project (after converting). Or the hardcore way: retopo, after it remodeling mesh model to NURBS and use it in CAD projects (veeeeery hard way).
And correction: not "solid" but another word (brep, cad model etc... ) Because STL file format was invented as description of solids for 3d-printing. So STL is also solid (if model is prepared properly ) :)
And another way: If you need to make big complex model for 3d printing (but not for industrial purposes), may be would be not bad to export assembly from CAD (Inventor, Solidworks, NX whatever), import it to blender and add there your STL. It can give you more chances to get something you need (of course with the price of lost parametrisation).
