Recently I had a plant identified in the ID forum by Ecrane as being Thulbaghia violacea. It is entered that way in the PF.
But on internet I found the same plant spelled in a different way as Thulbachia violacea.
I've always seen Tulbaghia (no "h" after the T) as the spelling here in the US. If Thulbacia is an accepted spelling, then I think you would just add it as a synonym (that's what was done with Buddleja and Buddleia). But the trick is to make sure it's an accepted spelling, and not something like one wholesale grower had it spelled wrong on their tags and now every nursery that buys the plant from them spells it wrong too.