I unpacked the DAT.vdk and had a look through it and found PossesionExp.csv (opens with excel) when opened it has the amounts of exp needed for it to level up. I had a little play around with values and repacked it, it changed my PB's exp % to 355099.65% but it did not level when I killed something.
Dunno if I am doing something wrong when I edit the values or if this just cannot be modified.
Location of PossesionExp.csv (must unpack the DAT.vdk):
If someone can take a look at this and see if it does work that would be great