AFAIK, it's not possible (Related How to setup K9 Mail properly for free email services offering multiple aliases for same email box?)
K9 mail isn't really active in development and I would rather suggest you switch to a modern Email Client which does this and much more. I am talking of FairEmail. Don't be discouraged by "unreleased" status. It's very much functional and actively developed. See this XDA thread.
Specific to your query see FAQ 9
I am not associated with the app but would happily recommend this a better alternative to K9 mail - it's leagues ahead of K9