Zammad does not support tree select like states.
Zammad provides on dimensional state lists - there’s no workaround.
I personally don’t see the benefit, rather sounds like you want to categorize stuff which should be done with a e.g. select or tree select object. (see: Objects — Zammad documentation)
If you still believe that this is a functionality that would be a good idea for Zammad core, feel free to create a feature request thread on this board. Please make sure to provide use cases.