recording keys into MPC

i have recorded a nice double bass loop into my MPC, can I now record some keys over this(my keyboard is not MIDI), whilst listening to the bass at the same time, or will I have to use an external sequencer?

I can record fine into sample mode but I want to hear the double bass as i (practise) playing along.....
Thanks...

yea you need something else because the 2000 and the XL cant do that.
The newer MPC's might be able to im not real sure though.

just record what you got so far onto something else(cd,tape,pc) then play it back while you sample your new keys in the mpc, the timing will be right ,then add keys sample to your sequence