Yes. Just keep in mind that the oldest (earliest expiring) expiration date from each of the funds will then apply to the new booking, and all those funds will be merged together and now carry that expiration date. This could be important if you later need to cancel that new flight.
Also, be aware that southwest limits the number of payment methods you can use on a single reservation. You may use a maximum of four forms of payment, which includes one credit card or PayPal account per transaction, along with Southwest gift cards, Southwest LUV vouchers, and/or unused ticketless travel funds. (The new booking system, for reservations after May 9, 2017, is even more restrictive, and limits the number of payment methods to three.)