So I know I might take some heat for this, but I like using Ableton Live's warping functions to process samples before loading into my SP-404sx. Last week I found this: slicing by grid.
http://loudonstearns.berkleemusicblogs.com/2012/07/09/ableton-live-slice-by-grid-and-audio-editing-tips/
By using an audio clip's launch functions you can chop a longer audio file by sixteenths, beats, and bars in a composition lane.
What I'll do after the separate audio clips are recorded into the composition lane is rename each chop to something like chop 1, chop 2, etc. Then I'll select all of them and crop. This saves them in the Live project file's Samples>Processed>Crop folder all with the file names chop1, chop2, etc. Then, with a card reader and the SP-404SX Wave Converter you can drop the chops right onto an SP formatted card.
Cheers!