If the sample is shorter than that, then you have to make a loop of it, maybe with a crossfade. First, go to the CRF knob and start adjusting it up until youre happy with the loop. Then go to the INS tab and make sure youre at the volume settings, VOL tab just below the INS tab. Then Set ATT, Hold and DEC to 0 and SUS to 100. Adjust the release until youre happy with the "aftersound". If you dont know what these things mean look it up from wikipedia or somewhere under "ADSR".
This method will allow you to sustain the sample as long as you keep the note playing.