That's not easy as it seems that's a lot of work! You can't copy and paste codes from pm templates and even if you get this working on newthread page there is other works to do in the thread creation process! Just to know why do you need this?
Why... you don't send new threads to people. You'd have to completely disable the ckeditor and add the custom code to it, or rewrite the vBulletin code to work inside of it (ckeditor).