Well, you can't convert an MP3 file to a MIDI file. I mean, I know you can find programs and apps out there which claim to be able to do this, but trust me, they are very limited in their ability to successfully convert an MP3 file to a MIDI file.
And I think I do understand what you're wanting to do-- play along with an MP3 song file-- but the PSR-OR700 does not have that capability. If you can find a MIDI file of the song you want to play along with, then that should work. But if you can't find a MIDI file of the song, but you do have an MP3 file of it, then the best option would probably be to connect your phone to your PSR-OR700 and play the MP3 file on your phone so the sound comes out of the PSR-OR700's speakers, then play along with the song while it's playing on your phone:
- Get an "auxiliary" audio cable that has a 1/8" (3.5mm) TRS plug on one end and whatever size TRS plug fits the earbuds or headphones jack of your phone on the other end (possibly 2.5mm, but check the user's guide for your phone).
- Plug one end of the auxiliary cable into the phone's jack for earbuds or headphones, and plug the other end of the auxiliary cable into the L/L+R aux in jack of the PSR-OR700.
That will let you use the keyboard's speakers for listening to the phone. Just make sure you silence the ringer! Or you can use a tablet, iPod, or other similar device that can play back audio files and that doesn't have any phone capabilities (so you don't need to worry about the phone ringing through the keyboard's speakers if you happen to get an incoming call in the middle of your performance).