How do I friend somebody with the follow feature turned on?

[John Smith]

You can just follow her for the time being. She will b notified that you follow her. People become a follow when the actual friend request was sent. If you accept it, you become friends, if you reject it, they remain a follower. Since you did not have followers enabled, they cannot be changed to a follower.
The only way you could do this is to accept the friend request and then to remove them as a friend, which will drop them down to a follower.