So my understanding based on the comments above is that you would like to parse the XML of the form definition in order to create POJO classes.
In this case, I think the right time to do this is when the form is published.
If you have your own persistence layer implementation you could easily hook up when receiving a PUT of the form definition when it is published. But other than that the problem is that there are no hooks for this at this point.
You could place a button in the form which only works at design time, but that would be quite hacky: you would have to write your own XForms, probably in each form you create, and hide the button at runtime.
If at runtime, then the XML of the form definition is not available.
In short I don't think you can do this without making more involved changes to Form Builder.