I know there has been some discussion on the Wall about getting from FRA to Munich with a Bayern-Ticket (RMV ticket to Kahl, Bayern-Ticket from there). I always tell people to get a Bayern-Ticket at the Regional Bhf from an automat, but I have only assumed it was possible. Now I know it is.
Today I did something similar. I went from FRA to Mainz with an RMV ticket, then used a Rheinland-Pfalz-Ticket to Treis-Karden. There are two kind of ticket automats in the Regbhf, the RMV automat for local tickets in the RMV (Frankfurt metro district), and the DB touch-screen tickets for all DB trains. On the opening screen of the DB automat is a touchpad for "Länder-Tickets". Touching that gives you a screen with a touchpad for each of the Länder-Tickets (Rheinland-Pfalz and Bayern, included). Easy. Probably took me less than a minute.