The method would depend on the shape of the part. Using some surfacing tools like thicken surfaces or "delete/patch" face could help in some cases. Using feature works (part of SWorks Pro/Premium) might help to translate geometry into SWorks features. If it is not a terribly difficult part, it might be quicker to use the STP file just to get the sketch geometry and model it using that.
It's a complex engine fan. I was hoping there would be a software to convert it to a solid.
When you import your part, check the "Options". It will probably help if you have "Try forming solid", "Perform full entity check and repair errors" and "Automatically run Import Diagnostics (Healing)" checked. If the part has errors, try using the Import Diagnostics healing functions. Sometimes you may need to try various combinations and orders of repairs. You can use the Undo function to get back to the beginning when you go down a dead-end path. Sometimes you may need to do your own repairs, deleting faces and using other surfacing tools, like Fill Surfaces, to get a usable solid model.