I would assume you would get the firstname and lastname values and set them to a string.
There's probably a better way, but this is how I would do it just throwing something together:
$user = get-qaduser "test"
$Firstname = $user.firstname
$LastName = $user.lastname
$domain = "newdomain.com"
$user | add-qadproxyaddress -customtype 'sip' -address "$Firstname.$lastName@$domain"