According to the implementation of RooUnfoldResponse::Fake(), it fills the internal _mes (measured) and _fak (fakes) histograms.
Given that _fak is intialized here using the “measured” histogram, in principle, what you want to do can be achieved by calling RooUnfoldResponse::Setup() or even this RooUnfoldResponse constructor. Try it and tell us.
Hi!
I have nothing to add to what @jalopezg already nicely explained, other than that RooUnfold is not part of ROOT itself, so support usually works via it’s own mailing list roounfold-support@cern.ch.
Feel free to get in touch over there with any further questions or requests!
Regards,
Carsten