Hello Marielle13
It is not possible to add tickets retrospectively to the SBB Mobile app.
The RhB webshop sells valid tickets; however, they are ‘travel agency’ tickets that cannot be synched with your SBB (SwissPass) account.
For now, you can just use the ticket’s PDF together with your physical passport or ID card.
For further ticket purchases, simply purchase from SBB. In Switzerland, we have a fully integrated ticketing system; this means, no matter which public transit service you gonna take, and regardless the operator of the service, you can purchase the ticket from SBB.
Kind regards,
Roland