Yes, if someone has already joined your group as a learner, you can still change their role to make them a moderator.
While in "Teach" mode, click "Settings" at the top of the course path and then select "View learners." This will take you to a screen where you can see everyone who has joined your class.
Next to the learner whose role you’d like to change, select the three dots, and then select "Configure registration." From there, you can change the learner's role.