Avast Thar Thumper,
Alas, there does not seem to be a way to get a true swish sound on a TD-20. However, to compensate, what I have done is edit a snare drum (using a different midi note number from my regular snare) and set it to a soft brush sound of lower volume and a slightly longer sustain. It takes some messing around, but doesn't sound too bad. I hope this helps.
Fair Winds and Following Seas,