At the moment, your best option is to create a magic item that gives you the proficiency. The upcoming character sheet revamp is supposed to be allowing on-the-fly adjustments like that, if I recall correctly. If I’m wrong, I’m sure someone here will correct me.
My DM gave me a proficiency bonus in game, how do I add another proficiency to my Character Sheet?
At the moment, your best option is to create a magic item that gives you the proficiency. The upcoming character sheet revamp is supposed to be allowing on-the-fly adjustments like that, if I recall correctly. If I’m wrong, I’m sure someone here will correct me.
-- Arms are for hugging The Dandy Warhols --
The workaround I use is Create backstory adding extra profiencies and feats.
Still need to be able to increase spell spots.
Thanks