Yes, you can add them after the 45-day mark. And Disney typically waives the $50 change fee if the price of your package/reservation increases (which it would, in this case). There may be a *minimum* number of days, but it's probably something like 5-10 days. (There's a cutoff number of days to add Magical Express for someone, too, which I think is a similar 5-10 days out)
Just be prepared to pay the entire amount of the increase in package price right away.
I've done this in the past when I've had a friend who was "on the fence" about whether or not to go. We left him off the reservation, and then when he finally made up his mind to go, we just added him. (If we'd had him on the reservation and then *removed* him less than 45 days out, there would have been the $50 fee)
-Rob