I always load the snare sample (if there's only one) into the NN-XT (I use it for all my drums) and copy it a few times, so it'll run from say C to F. Then I detune them all slightly - not very noticeable, but just so that it doesn't sound like the same sample on repeat. That way you can use two fingers and you don't have to be fast enough w/ one hand. But, if you still can't, just overdub it - play the first notes or whatever you can, let it loop back to where you were at, play the next notes that you can, overdub, repeat, until you finish the roll.