So what you basically ask is a plugin for a plugin... hmm... The most simple way to create custom nodes like is done for the ESB node (look in the source how it is done), but I'm afraid (yet no expert) that you do have to 'change' the basic GPD.
well I'd like not to change the GPD plugin. For what stands the ESB-node? In the documentation(http://docs.jboss.com/jbpm/v3.2/userguide/html/processmodelling.html#nodes) I can't find anything about that node :(
As I have replied in the other thread, it is best to checkout the code of the designer, look at the provided extension points and see how they are used by the designer itself.