Hello Zomoocaish39
It is not possible to add tickets to your account that were previously purchased as guest.
Whether you can change your tickets, depends on the fare that you have booked. For example, non-flex tickets cannot be changed at all.
To change the tickets, you need to call SBB, there is no other way to change the tickets, if at all. Interestingly, we often meet U.S. residents on our platform who do not know how to make international calls. I will tell you how, simply call: 011 41 51 225 78 56, available 24/7. And yes, the people at SBB do speak English, just as we do here in the SBB Community, a customer support forum run by volunteers.
Kind regards,
Roland