Since both STEP and 3DP formats deal in 3D graphics, it is likely they can be - at least partially - converted between each other. However 3DP format is used for "3D" photographs so a STEP file as input would probably not yield you the desired output. So probably possible, but ultimately fruitless conversion.
Convert ISO-10303 STEP product data to Autodesk 123D photo scene file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert step to 3dp on Windows
3D viewer and converter with huge file type support
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.