They can't reverse sex themselves. If yours has already morphed into a female, which I would assume is highly likely since you've had it for over a year now, then puting another female in the tank will cause problems.
I haven't heard much about Oscellaris clowns being aggressive, but have been told that Oscellaris are one of the most easy going clowns available. I would just put a small clown in there and eventually the 2 will co-exist.
However, I'm still a novice to all of this but have been doing a fair amount of research on clownfish over the past few months. Hopefully someone else with more experience can chime in. But no matter what, don't put a larger clown in there. Always better to go for a smaller one, if not the smallest possible just to avoid any chance the 2 are the same sex.