What I do is: when I'm done laying out the GUI, I save the FormBuilder file, and generate the file containing the inherited class. Then I copy the inherited class file to a separate working file. I then edit the working file to subclass the main class from the inherited class file. I can then edit the working file as necessary to add event handlers etc. but it picks up the FB GUI instructions.

If the GUI needs changes, I change it with FormBuilder, save the FB file and regenerate the inherited class file. This, then, remains subclassed in the working file. The GUI is updated, but the working file is unaffected.

This has worked well for me.