Yes, it is very easy.
Through the web interface, logged in as admin, go to the domains section. Tick the box next to the domain you want to move to a new client then click "Change Owner" at the top.
After selecting the new owner, you will be presented with a new screen which you can change the IP address for that domain. Note: the new IP has to already be in the users IP pool, so do that before moving the domain.
Select the new IP from the drop down box, click ok and all done (hopefully).