So, don't "usually" become masters until, means that you don't actually have to do that to be a master. Even if she is not a member of the order, she is still trained in the Jedi tradition, so should be able to progress in it.
So we have someone who while she isn't a member of the order, is a member of the tradition, has achieved the rank of Knight, even if she quit before it becoming official, and doesn't need to train anyone to become a master.
Sounds like she could take the levels if she wanted.