I'm not a keyboard player but I've worked with some.
I would assume that the keyboard is midi capable, in which case one should be able to download midi files into memory. There are some free midi files available on the web but you'll need a laptop with a midi interface.
From the Roland web site: "Sometimes you need a very specific sound or the unique capabilities of a particular soft synth or sample library. The RD-88 has three zones that can be configured with internal sounds, or can integrate and control soft synths from Apple MainStage. This makes it simple to duplicate the sound of a track live or mix favorite soft synths with the RD-88's own palette of sounds." Sounds like the RD-88 is designed to work best with a Mac and the Apple MainStage software.