In the Mix/Layer window you can set the sample offset, but it\'s not controllable.
I see two options:
1) Set the offset, and then modulate the attack (In the EG/Mod window), or
2) Set different layers with different offsets, and then switch or fade between them. If it\'s a solo instrument, definitely switch. If it\'s an ensemble, you may be able to get away with a fade, though the phasing effects may detract. I\'d go switched.
Setup the samples to have many velocity layers, say eight. Each one can have it\'s own sample if you want, or you can use the same sample for multiple velocity layers.
Now select each velocity layer one at a time, and adjust the sample offset for each layer differently. Should work really well.
If it\'s a mono sample with no other layers, you can do 32 velocity layers. If it\'s stereo, you get sixteen. That\'s probably overkill. And more work than you need to do. Eight should be pretty seemless. And four should be reasonably effective.