What follows is a tried an tested method to create a Parasolid from an STL file using Mentat and MSC Apex.

Summary of steps:

1) Import the STL into Mentat using the FEM option (Target Entities)
2) Export a BDF file
3) Import the BDF into MSC Apex
4) Using the "Geometry from Mesh" option, create a solid
5) Export the solid as Parasolid file.

Detailed Steps:

1) Import the STL into Mentat using the FEM option (Target Entities)

2) Export a BDF file

3) Import the BDF into MSC Apex (ensure to choose the correct units of the STL file)

4) Using the "Geometry from Mesh" option, create a solid with these 3 steps:
a) Select the body and click the Middle Mouse Button (MMB)

Note that the above is still only a preview of the geometry features which can be edited before committing to the result.

b) Select the 3rd step and switch on "Create Both Facted and NURBS Geometry"

c) Execute the step by clicking the MMB or the green check mark.

Note the solid that now exists under the part in the model tree.

This solid can now be modified just like any other solid, like using Push/Pull, Defeaturing, Splitting etc.
5) Export the solid as Parasolid file.