Exactly, right now the form builder is being designed to help us in design time, that doesn't mean that in the future it can do what you need. Right now the form builder is being merged and continously improved. You should contact mariano to see here the source code is and how you can participate with it.
https://github.com/marianbuenosayres/jbpm/tree/master/jbpm-gwt/jbpm-gwt-form-builder
Cheers