Can I be added to an existing mortgage?

My boyfriend bought a house with the help of his parents. Both of them
are on the mortgage. My boyfriend wants me to move in with him after
engaged. Is it possible to add me to his mortgage, if there are
already two people on it? I dont want to move in and help pay on a
house that I cant count as mine own as well if we are to build a
family in it. And having his parents off of the mortgage doesnt seem
feesable because his dad lives there part time when he works in town
and pays part of it. Whats the best way to go about this? Help!Many
thanks,
Glamour2c