I don't own a S900, but I was under the impression that such bass slides could be found in the four MegaVoice bass voices (last four under 'Guitar & Bass' category).

Can't one use one of those voices with velocity fix (set to the velocity value the slide is triggered) if such available?

If velocity fix is not available maybe with 'touch sense depth' (set to 0) and 'touch sense offset' (set to the velocity value the slide is triggered).

Shiral