Technically it’s possible, but it’s a pain. You’d have to write a script which logs in to your control panel as if it’s you and does the setup for you. Unless you’re going to be giving out tons of subdomains every day, I can’t imagine going to the trouble to automate the process.
A while ago when I ran Jabber, I wrote a script which used that technique to automatically set up a chat account for every new user who registered at my site. It worked, but I would have preferred a more elegant solution. (And before anyone asks, sorry, but I can’t share the script. I don’t use it anymore and don’t have any copies.)