This is not possible through the SBB ticket shop.
You can buy the reduced tickets through the Jungfrau Railways ticket shop, simply select _Swiss Travel Pas_s: Buy Jungfraujoch Ticket | jungfrau.ch
A personal note concerning buying tickets for mountain excursions in advance:
In the Swiss mountains, the weather plays an important part. It is advisable not buy tickets in advance as you might meet bad weather. Keep your travel plans flexible to choose another day with better weather or buy the tickets provided the weather forecast is predicting good weather, this is usually not possible more than 2 days out. There is no need to buy the tickets in advance: you can buy the tickets on the day of travel at the station in Grindelwald or Grindelwald Terminal station, too.