Everything points to the fact that no cadpart to stp conversion possible. Nonetheless, it might be possible that you were in fact looking for a way to convert CATIA part designs (.catpart) to ISO STEP product data format (.stp). In this case please check out the catpart to stp conversion entry.
In a word, direct cadpart to stp conversion is not possible.
So don't expect any so-called cadpart to stp converter or cadpart to stp free online conversion tool available.