Ah, I see what you mean now!
BTT cannot add or edit the native touch bar set untill apple allows it (probably never).
You can however, change what the BTT button does. (This is a reletivley new BTT feature, update if you can't get to it. It also may be buggy too but it's worked well so far while I use it everyday)
Add a Control Strip Button widget:
This will replace the BTT toggle button with whatever you please. It seems very fitting too as you only need one button to add!
You can also edit the icon, add an action if you hold it, etc.
Hope it worked!
p.s. I suggest you try out my preset: AquaTouch 
(I'm working on outlook support coming soon too)