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 have found one software suitable in our database for this conversion.
Microsoft Windows software - convert step to 3dp on Windows
3D Object Converter is a powerful 3D polygon model translation and interactive viewing shareware tool that allows arbitrary 3D model data to be imported from external sources, exported to various industry standard 3D file formats with great accuracy and quality.
Convert from step file
to 3dp 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.