@srh11, when you say "I have the mesh of a specimen in STL format" I don't think you mean a finite element mesh, you mean the faceted geometry of the specimen that is used for printing.  It doesn't help when you add "This mesh is made of shells", again, I don't think you mean finite element shell elements, but I don't know what you mean.

ANSYS SpaceClaim Direct Modeler (SCDM) is the default geometry editor that comes with the Student license and it can open STL files and represent the facet body.  There are tools to convert the facet body to a solid body. However, the Student license has a limit of 300 faces, so you can't save a direct conversion of STL facet body to solid body because you will exceed the face limit.

If you have an Academic license provided through your University, then there is no face limit and you can use SpaceClaim to edit the solid body, merge faces and generally clean up the geometry so it is suitable for meshing the solid body.