Hello Philipp,
There is perhaps a way to get what you want, but it cannot be done
solely within the single form that you have now since the Form is
designed to handle common/simpler use cases.
Since you have control over when events are dispatched to the form, you
could intercept a relevant event (combined with checking the form state)
to decide when to pop up a dialog on top of the form. This would make
it possible to implement a modal dialog box (even with its own form for
data entry, as you described). Once you are satisfied with the data
entry in the dialog box, it would be closed and you'd be able to take
the resulting data and rebuild your form with the new data inserted.
The devil is in the details, naturally, and if you'd like to discuss
this further then I'd recommend opening a ticket where we can hash out
the details, see code samples, etc.
I hope that helps!
--
Jonathan Daugherty