It's trivial.
In GlobalDefines.xml, there's an entry
BARBARIAN_MAX_XP_VALUE = 30
Either set it to a huge value (9999), or possibly to -1 (the game seems to generally recognize -1 as being a way to disable a limit).
Plenty of other good things in that file to mod; for instance, CIRCUMNAVIGATE_FREE_MOVES is currently set to 0, so being first around the globe does nothing. One of the first things I did in my mod was change that to 1.