The way I'm understanding this is that you want to create a Primary Tag, have tags under that primary tag and add tags to those secondary tags like XenForo/Vbulletin has it structured.
So lets say:
Video Games (Category) (Primary Tag)
- PlayStation (Forum) (Secondary-Primary Tag)
-- PlayStation Store (Sub Forum) (Sub-Secondary-Primary Tag)
-- Help (Sub Forum) (Sub-Secondary-Primary Tag)
-- General Discussions (Sub Forum) (Sub-Secondary-Primary Tag)
From what I tested, Flarum can only have one Secondary Tag under a Primary Tag.