My husband is flying separately to our destination, but on the way home, we are taking the same (2 flights) home. Right now we are on 2 separate reservations. Since I'm about to earn my companion pass, I'm hoping he can cancel the return flights on his reservation and then I can add him to my current reservation as a companion just for the return.
The short answer is yes, but when I have done this in the past, I had booked each direction as a separate reservation. If you are booked as one round trip, I would call SW to get an agent to help you accomplish this change.
-A List Preferred, Companion Pass holder, Community Champion.
@WilDog one of the really nice things about flying with Southwest is that there is not a surcharge for booking one-way flights. When you book a round-trip it's essentially the same process and pricing as booking two one-way trips. You'll see many folks here in the discussion forums that book one-way trips to give them more flexibility in a situation like yours -or- to give them flexibility to change one of the legs but keep the other (another benefit of Southwest is no change fees). Enjoy your CP!