Honestly, I'd recommend not doing that.
The QT is a place for new fish, who potentially have diseases, to come in, recoup, relax, unwind, and get treated as necessary. If you have a single fish in the tank, that fish is going to get used to being alone. It won't take too kindly to additions. And, you also don't want to expose it to diseases and medication treatments. New fish need peace and quiet while in QT. They don't need stress from other fish.