Yes, it's easy!

Just tap "Add Existing Gift Card" on the Pay screen.

If you have cards in the app already, tap "More" at the bottom of the screen, then tap "Manage Cards" in the menu.  Tap "+ Add Card" button at the top of the screen. Then, just tap "Add existing gift card" and enter the 19-digit e-card number and PIN code as you would do if it was a physical card.