Hopefully I'm not reanimating a too-long dead thread, but this is the only most applicable result that google gave me, so I wanted to post the solution I found here for future wanderers.
I am just starting to use Deckbox, and have tens of thousands (est) of cards to enter. I would like to be able to enter the cards that are in my decks, but I don't want to individually re-enter those cards into my inventory as well, or vice versa. The best workaround I can figure out is to enter the deck as normal, and then export the decklist as a .CSV using the button at the top of the page. You can then go to your inventory, import the .CSV to add the cards to your inventory.
However, this only inputs the name and quantity of the cards in the deck. You will have to add all of the other information (edition, condition, language, etc.) once the .CSV is uploaded. You can do this by adding and using a filter to your inventory results; I filter by "No Edition Set" because I'm careful to mark the edition of the cards that I manually add to the inventory, and can trust that only the cards imported from the .CSV make it through the filter, and then I can click each card individually to add the ancillary information I want to include.
It's better than nothing, but it's a cumbersome process and pain in the butt. It (barely) beats having to manually enter the cards both places, so I'll probably end up paying for a premium subscription to use the "Add to Deck and Inventory" function.
But if you need a way to do it for free, this is probably as good as it gets. Happy inventorying!