Any drum pad that has MIDI connectivity
shouldn't have any problems accessing the CVP's internal sounds and vice versa, however, if you want to go with a Yamaha, there are at least 3 drum pad models that would go well for what you're trying to do:
* Yamaha DD55 & DD55C
* Yamaha YDD-60
You would connect the drum pad and CVP with MIDI cables. In your scenario (you want to control the CVP with the drum pad), you would attach one end of the MIDI cable to the drum pad's MIDI OUT port and the other end to the CVP's MIDI IN port. At this point, the two instruments should be communicating with each other. If not, consult the instruments' manuals to determine what parameters need to be set (such as local channels, etc) in order to enable communication.
Hope that helps!