You can buy more styles from Drumtrax.com. Will cost you $25 which I think is a reasonable price, not much more than a music cd, actually.
I also recorded some drum rhythms from my keyboard. Took one style I liked particularly much and sequenced it on the keyboard to go through all variations and fills. Then I edited it for Session Drummer as described. I actually made a couple of extra short fills by simply deleting some drum beats (mainly snare ones) in the fills.
I know this is not as intended, the idea of Session Drummer being that it is the recording of a real drummer and hence sounds more true to life than the usual midi drums. But the keyboard style I recorded was some disco where you do expect a "drum machine". Just nice to have it in Session Drummer to experiment with it directly.
... does take some time to do, though, particularly because of problems with sync'ing the keyboard. I had to record small slices only and often repeat the recording until I suceeded.