You're half mistaken--GDRB certainly can be exported; many, many of us have done so. (You're right in that TBRB cannot be). See the details of how to do it here. I never actually played GDRB; the disc was only in my system for long enough to do the export.
Conceptually, exportation is not into one game or another, but from the game disc onto your HDD, where the songs can be played in other games. Having exported the songs from GDRB you can play them in RB, RB2, RB3 and Lego (the "Family Friendly" rated ones), unlike the versions of the songs newly added to the store, which can only be played in RB3.
Last edited by mikeyts; 11-29-2012 at 09:43 PM.
Mike Scott, San Diego, CA, USA (XBL: MikeHellion, PSN: MarcHellion)
Last edited by Bront20; 11-29-2012 at 06:09 PM.
<Insert request for more Boston here>
PSN: Bront20
DLC: lots+RB1+LRB+RB2+ACDC+GDRB (+ RB3)