It is on SD7, so almost certain you can on SD90. You should be able to open the MP3, call up the PDF and Save, but Ketron sometimes has a tricky way of doing things. Try calling up the PDF in Player 1, and the MP3 in Player 2 and save to Registration.
Or make a registration with the MP3 in it, THEN go to a Player and load the PDF and then Save.
I'm sure AJ knows how exactly.