Hello Thoopeekue58
Yes, tjis is possible since the Swiss Travel Pass is vslid for all trains in Switzerland.
You can book a ticket from Milan to Bern, and declare the ‘GA Travel Pass’; by doing so, the system will book you a ticket from Milan to Bern, but you will only pay the part from the Milan to the Swiss border (Domodossola). For Domodossola to Bern you can show the Swiss Travel Pass.
Alternatively, you can book a ticket to Domodossola and simply continue on the same train using the Swiss Travel Pass, both is possible.
Kind regards,
Roland