SIS,ET, W?SÁNE?, or SEN?O?EN or, if your ideas page supports:

SÍS,ET, W̱SÁNEĆ, and SENĆOŦEN

In my use case, I wanted to add a rôle (e.g role) with character(s) respecting Canadian First Peoples (because Grandmother won't tolerate that foolishness).

I appreciate (from a Canadian: no higher honour is bestowed) your candid response:

To answer your question: no, the custom role fields can't accept SIS,ET, W?SÁNE?, or SEN?O?EN with their proper characters right now. This is a current limitation, not something on your end.

P.s.: Whilst I am "signing up" for a Canny account, why can I not submit an idea as a paying Digital Ocean customer using my login (as Grandmother says, "I don't work for you, so why do I need yet another social media account".)?