To get RB2 wads to work on RB3, For me all I had to do was;
Follow the instructions to install wads to the SD card.
Run wad2bin, use the parent ID for "RB3", select ONLY the RB2 DLC wads. Check 'nullkey'.
Move the WAD file(s) starting with “00010000” and ending with “bogus.wad” to a folder...