This question is frequently asked in our English-speaking SBB Community: Questions and answers - SBB CFF FFS Community
There is a straightforward answer to your question: no, this is not possible. You cannot add tickets bought as a guest to an existing SwissPass account. You would have had bought it while being logged-in.
Kindly note, tickets purchased via your smartphone are personal and non-transferable and are only valid in conjunction with a valid form of ID belonging to the person travelling (passport, identity card, driving licence). More information on the SBB Mobile app can be found here: SBB Mobile | SBB help & contact