It appears that you can render models saved in STEP 3D (.stp, .step) formats to Pro/Engineer's .slp files, but for this you will either need the original software, it's successor or some kind of 3rd party conversion programs would be capable of this procedure.
Convert ISO-10303 STEP product data to Pro/ENGINEER rendering file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert step to slp on Windows
A 3D CAD translation softwate
Convert from step file format
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.