returning to the original question:

if truly STRETCHING a clip is what you want to do (all previous caveats still apply, I'm not saying its a good idea, just saying how to do it in RB)

1) hilite the clip

2) right click on the track to open the right click menu

3) select PITCH SHIFT AND STRETCH AUDIO <-- may say something a little different, my memory sucks

4) uncheck PITCH SHIFT

5) enter a percentage of stretch for the clip. For example, if you want a 2 bar clip to fill 8 bars, enter 400%

It will probably sound pretty bad though... works best on sounds that are already highly sustained pad-type sounds... because no matter what it was originally, after stretching, its gonna sound like a pad