It's not really what they are designed for.
The MPC was a sequencer first, sampler second. Only recently have they been marketing them more as samplers than sequencers.
Still, the sampler in an MPC is very basic and is definitely overshadowed by the incredible sequencing power.
You could simulate this two ways:
1) have the samples you want to play already in the machine, and build your sequence up live.
2) use MIDI instruments, and build up the song track by track...in this respect, the MPC will become far more powerful than any looper pedal around today.
For an example of live MPC looping, dig Anchorsong, he is a master of live sequencing the MPC 2kxl.
Here he starts out doing what I mention in #1, then moves on to doing what I mention in #2 -
http://www.youtube.com/watch?v=cEW4iP5LsK8http://www.youtube.com/watch?v=qXMJ49_M ... re=relatedhttp://www.youtube.com/watch?v=Lf5w_NlP ... re=related <-best one