One note however: The “GA Switzerland” reduction means that you buy a ticket at normal price, with the Swiss Pass zero rated. If you buy a ticket Domodossola - Milano you may end up paying less, as you then get access to discounted tickets, and special offers like 2 for 1.
So I would just do this: Look up the schedule you want to travel on sbb-tickets.cc. Look for solutions with a direct train from Spiez to Milano. Then look at the train details, and look up when that train stops in Domodossola. Then use that info to book the EC from Domodossola to Milano.
You will travel till Domodossola on your Swiss Pass, and from there onwards on your Italian ticket. Easy. No need to get out of the train, and you can probably already take up your seats in Spiez. If not, take a seat somewhere else, and change when your booked seats free up.