Hi John,
.....Glad to help Just a suggestion - If you first make a sequence of a song, on your keyboard and either leave off the full melody line, or part of the melody line, you could play the guitar part along with the sequence. I'm not familiar with the S900 so I don't know if it has a suitable audio input for your guitar, which would allow you to mix the guitar and keyboard sounds and then pass them from the keyboard audio or digital output to your recorder.
Some time ago, I experimented using this basic idea on my KN7000, ie using the KN7000 to mix the sequence and guitar parts, and then recording to PC via USB. The results were quite acceptable.