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'd be glad 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 contribute to designer at some point something you think would be useful to have.
Many thanks for the quick and usefull response.
I'll look into it and if I do get to the point of asking for help on irc I'll certainly try to make a contribute.