Starting with v9.0.0 UrzaGatherer will let you add cards really quickly in a defined deck.
To do so you need to turn on the feature in the settings:
Then when in collection mode you will have a new icon on each card:
By clicking this icon you will add that card to the Active Deck
To set up which deck is active, you will have a new icon in the footer (just on the left of the Settings page icon):
By clicking this icon, you will be able to set the Active Deck:
I hope you will enjoy this new feature!