Quick answer: Yes this is possible with creating a new profile. A profile determines what stencil set to is used among other things. A stencil set is combined of a number of svg and png images of your nodes, node attribute information, connection rules etc.
As far as if it's difficult - not difficult but it does take time to develop your own profile.
I would be willing to help you out with details (hit me up on irc - http://www.jboss.org/jbpm/irc), but would ask that once you get to the point where you are familiar with the designer code (which you will be after creating a new profile) that you do contribute back with something too ;)