Apr 11, 2016 - 2:40 PM
Hi BobbyN, welcome to MySwissAlps!
In addition to Brahms' answer: you could use an IC train that doesn't require seat reservations for the leg Brig-Domodossola, get out in Domodossola and board the train you've made reservations for in there. In all, this might be easier the easier option.
Alternatively, you can use a the direct EC train all the way from Brig, but that one indeed requires seat reservations. Since the leg Domodossola-Stresa is already covered, you additionaly need seat reservations for Brig-Domodossola. Mind that you'll be traveling with two different seat reservations, so you'll probably have to switch seats halfway. You should be able to do this using the SBB: enter Brig-Domodossola, pick the exact EC train you have the reservations Domodossola-Stresa for, and enter the buying process. When asked for reduction options, select "GA" in either 1st or 2nd class as this provides free traveling for Brig-Domodossola, just like your Swiss Transfer Ticket or Swiss Travel Pass (I'm not sure which one you'll be using) will. This way you'll pay for the seat reservation only, not for the trip itself. If this doesn't work, please refer to the resellers listed at www.myswissalps.com/tr aintickets/italy/price and pick "seat reservations" from the drop-downlist at the right.