I suppose this could be a bug? According to the wiki it’s supposed to be possible (fig 7)
It partially works… until you actually try to use it… I’ve made a descriptive image to better illustrate what’s happening:
For the record: This is also happening with layers. The moment you try to create a new layer, all of the layer slots become duplicates of the first one.
Oh please let there be some sort of workaround for this… 
it’s a mystery eh? Surely I’m not the only person on the entire planet who customizes my ZBrush UI??
Note to pixologic: I know you guys need beta testers to crank out fancy art that you can show off new features- but if you’re ever in the market for somebody to go through and test all the OLD stuff to make sure it still works (We call this regression testing), please let me know. I’d be happy to oblige 
This wasn’t possible in ZBrush 4 either. I don’t know if there are plans to make it work in future versions or not.
must be nice to be able to run a business like that…
‘oh hey this thing broke… lets just leave it in the wiki and never fix it or acknowledge that it broke in the first place. we’re pixologic, who cares?’
blargh… so frustrating.
totally agree, I spent an hour creating 4 custom menus with the tools and menus I use the most and after I hit store UI…and saved off a version just in case…it wouldn’t load it back up…the menus at the top are there…but only 2 of them have something in them…the rest have nothing…woohoo…fun… :’(…