It seems that this conversion represents a translation of 3d graphics saved in ISO STEP format (.stp, .step) to a Universal 3D format graphics (.u3d). Apparently, this conversion is possible and can be done with some of the dedicated 3D conversion tools, such as PolyTrans.
Convert ISO-10303 STEP product data to Universal 3D format graphics.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert step to u3d on Windows
A software suite used for 3D CAD/DCC + Animation/Model/NURBS/Skinning translation, optimization and viewing.
Convert from step file
to u3d file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.