I draw the flow out and such in the gui, and create the original variables in the gui, then I copy/paste over the variables later down the road for the others nodes because its faster. So if you wanted to sit there and type out the xml then copy it over, it works unless its not correct because I have done it myself.
Extenalizing the graph creation code from the GPD is not really possible I am afraid. It is quite deeply tied into the Eclipse GEF framework. I for sure would not know how to use this code outside the Eclipse plugin. If you find a way to do this, let us certainly know...