You can turn any unposted reply into a private message as seen below:
Here you can see me adding a user to the recipient list.
However, when I turn the private message into a reply and back to a private message, all the recipients are lost:
I’d love it if doing so wouldn’t wipe the recipient list. It’s a minor change but will save time.
[I am unfortunately not able to add more than one image, I apologize for the links.]