I’m not a user of Abaqus, but take advantage of the hobbyist (free) version of Fusion 360. The program enables one to import a mesh file (STL) which one then converts to BREP. Once in BREP form, the many triangles of the typical STL file can be merged into a contiguous face (depending on the overall geometry).
Your question references having an STP file, which Fusion 360 will export. If you require the STP file to accomplish your goal, the objective may be reached in this manner.
Consider also that STL files may not be manifold and if so, will not properly process in many ways other than that which you describe.
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