The only way I know of is a tool in fusion 360 fat automatically converts meshes into solid objects within the program by combining the triangles into faces and feeling everything within those faces in. I'm not sure what all of these guys are yapping about with there being no possible way to turn a mesh into a solid object, but I will say unless you have a very good PC it's likely that this process will fail if you're trying to convert a complicated mesh. But personally I think these guys were just being NPCs and not realizing what you meant. Instead choosing to thank you meant a literal file to file conversion. Oh also PS just to make sure I wasn't being a dick about them not knowing about something that didn't even exist yet, sp I went ahead and looked up when they added this feature it was 2013