We can't do that on Keane's forum either (same phpBB board type) - I don't know if it's just that it hasn't been set up or that this type of message board system doesn't support that type of thing. Oh, how much time I would have saved if this were an option on these message boards...
I just cut and paste the main message and then do the rest manually. I also use firefox, so when I PM someone once, the name comes up as I spell it, so I never have to type it out all the way again.
But it would be really helpful if there was a function for this.