You can mod the "black_market" "backpack" SDU to a higher number then 6. OR if you have a lot of items on another save and you want to combined the two onto one. You can extract them all with python python savefile.py -e items.txt your-save-game.sav An add them all into one by editing the items.txt. By editing I mean c+p them all into one items.txt Add them with python savefile.py -i items.txt old.sav new.sav Here's a link to the python file I reference https://github.com/pclifford/borderlands2
have you tried this> python savefile.py -m unlocks=slaughterdome old.sav new.sav but for the new "slaughter dome" for The pre sequel now called Shock Drop Slaughter Dome, just wondered if you'd tried pat i'll have a look when i get home but I think it should work
Heres the link to the file you use with python. As for downloading python that's a whole nother process. https://github.com/pclifford/borderlands2/blob/master/savefile.py