You have a few options. One is to setup the Caller System transfer as an option off of your main AA menu. Just be sure to edit your Restrictions table in Unity to allow those numbers to be dialed from Unity. This will allow you to dial any extension, including non-VM subscribers.
Another option is to just be sure the key(s) aren't "locked" on the caller input screen for numbers that are valid Unity subscribe extensions (ex. extension is 2345 so make sure 2 isn't "locked"). If you have multiple DN ranges like 2XXX and 3XXX make sure that 2 and 3 aren't locked. This will allow anyone to dial a subcribers extension any time during the AA greeting.