Yes it is possible as Domodossola is the border point. No need to change trains as well, simply stay onboard :)

Keep in mind that you need to add seat reservations in Italy :

- 13€ Frecciarossa

- 10€ EC trains

- 3€ IC trains

Those are best bought on tickets.oebb.at (bo booking fee) : add Interrail/Eurail as a discount and select one-way tickets.

Seat reservations are not a thing in Switzerland.

That is possible (and a reservation Milan-Domodossola, depending on the train). But if Milan to Domodossola will be your only journey in Italy, then just buy a regular ticket.

Hey @thibcabe thanks for your reply, so what I understand is that I can use both my passes and stay on the same train, same seat with just reservation till domodossola without requiring any reservation for journey further and it won’t be a problem crossing the border. 
 

hey @rvdborgt thanks for your reply, I will get back from Switzerland and have to travel further on in Italy. So opting a Eurail one country pass would be my best choice I guess.

@thibcabe can I do the same while returning back from Spiez to Milan Centeale?

Yes obviously :)

Great Thanks @thibcabe