Importing an STL 

• Select Insert → Insert Mesh

...

• Click the Center and Move to Ground buttons.

• Click OK

Generate Face Groups

From the Mesh tab select Generate Face Groups

...

You should see a multi-colour object if things worked out.

Convert Mesh

From the Modify pull down, select Convert Mesh.

...

Note: This does not work on any curved services.

You now have a solid object!

Reference