Originated system knows much more about the model, so that generation of UV coordinates would be better done there. Unfortunately, STEP Recommended Practices for tessellated geometry puts completely out of scope texturing and defines only vertex normals:
The following is out of scope for this document:
The definition of Tessellated PMI Presentation.
The definition of 2D Tessellated Geometry.
The way to manage alternative shape representation (exact and tessellated, levels of details).
The definition of textures, colors, normal on points.
The definition of scan results.
The definition of scenes.
Therefore, it would be better using another format for exporting triangulation from originating system (like glTF, OBJ or similar) instead of STEP. Or export B-Rep instead of tessellated representation to STEP file.
As for dealing with what you already have - OCCT doesn't provide any tool for automatic generation of UV coordinates for existing triangulation.
The first search in the Web suggests using xatlas C++ library, which is able to generate UV texture coordinates, but I've never used it personally.