Is there a way to add more than one number to iMessage?
Yes, there is a way to add more than one number to iMessage. Just establish cellular service and SMS capability from any carrier you choose for each phone number you wish to have for iMessage. On each iPhone that has that service, sign in to iMessage.
I've had three numbers with one Apple ID in the past, so it wasn't a problem other than the obvious one of having three phones.