There are some ways this can be done.. but previous posts mention important items. STL files define triangles (and an orientation), but neither units nor any other kind of geometric information.
My company developed the InStep Studio 3D application (feel free to google that) which can convert STL to STEP and in general there are three different ways of doing that from within the application.
The first and most crude (but sometimes also with the best result) is the direct conversion where each triangle is converted to an equivalent trimmed surface. Sometimes (especially with complex and highly detailed shapes) reducing the number of triangles is required (and achieved very easily). A loss of detail may be the result of this – depends on the data.
The second option is to attempt a feature detection. As mentioned, most CAD intends to use standard geometric definitions to define the body. Our application can find common items (though this is still under development).
The last option, especially for highly organic shapes (faces, things with smooth surfaces) is to overlay a grid that is then used to define a smooth (NURBS) surface. This approach can be hit and miss and greatly depends on the shape in question and the extent the user is prepared to go to.
If you can post your file, I would be happy to take a look and perhaps do the conversion for you….
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