I would like to do a motion study analysis in SolidWorks, but I wonder that can I do the analysis on a step file? I am asking this because the mates are not available on a step file.
No mates, no motion, or at least correct motion.
You'll need to apply new mates to your assembly.
Best thing to do is save the new assembly and all the part files that come with opening it. You will unfortunately have to re-assemble the assembly, but for accurate results, it's what needs to be done.