Yes, you can if you have booked an e-ticket - it is super easy!
Head to 'My Tickets' and click on the journey you wish to add to your wallet. This will open the e-ticket in your app. You will see the button to add it to your Google Wallet at the top above the ticket, click this and it will add your journey to your Google Wallet.
If your ticket is a return ticket you will need to add the outbound and return tickets separately. To do this make sure you have the ticket you are adding, visible on the screen before selecting "Add to Google Wallet".