In the deck, go to View > Print. Copy that list.
Go to your Inventory/Tradelist/Wishlist and go to Import > Card List. Paste in the list. Click Import.
that's all well and good, but what if your deck is made up of cards in your inventory, and cards not in your inventory?
ideally, I'd want Deckbox to have the option to add the cards in the deck that aren't already in the inventory/tradelist...