Am I correct there is no way to grant proficiency in "secret languages" (Druidic, Thieves Cant) outside of taking those classes?  I wanted to create a "Criminal Adept" feat that granted proficiency in thieves' tools, a rogue skill, and Thieves' Cant, but that doesn't seem to be possible.

As far as I'm aware you can't add Thieves' Cant to a feat, you can however just go in and edit 'Proficiency & Languages' to add Theives' Cant. Extra steps, but doable.

Thieves' Cant isn't in the languages dropdown, but I guess you can just create a "custom language" and add it.