Since it is an open source product, of course it is possible, if you want to edit and compile the GPD.
However, that being said, you need a fair amount of experience with numerous technologies to rebuild the GPD. Eclipse plugin development, SWT, GEF, etc.
The current state does not support custom nodes, but that would be something great to see in the future :)
Carefully study the plugin.xml file of the org.jbpm.ui plug-in. You should be able to find out how to use the extension points to add your own custom node to the designer.