Yes, you can.
Log in with your SwissPass profile under More -> Profile. Based on the login and the information stored there, the BLS Mobile app recognizes that you are in possession of a Half-Fare Card. Under More -> Profile, it shows you your Half-fare Card in the top left-hand corner under your name and date of birth. This allows you to purchase your tickets at a reduced price.
If your Half-fare travelcard is not linked to your SwissPass login, you can select your Half-fare travelcard under More -> Profile.