Can I use my Buff points to gain more Battle Pass XP on Overwatch?

Indirectly, yes you can. If you've got a bunch of Buffs stacked up, you can use them to claim a Blizzard gift card ( if they're not in circulation on the day, get an Amazon gift card and use it to get a Blizzard one,) then use that to redeem some Overwatch 2 coins. You can use those to buy levels in your battle pass to catch up on your progress!