You could create a ghost deck using any card you want from a unique GUI. That deck will be a new feature which will be only intractable while in the forge.
It won't bare any conditions the original deck's functionality bares, It will be almost visually only, however, you'll be able to summon the creatures from the deck into the forge's battlefield.
My suggestion is:
Adding another "Deck" with different color right above the original player's deck. That deck will be a "Ghost" deck, a deck which bares no conditions when attempting to find a match, trading and will only be seen while in the forge.
In addition, a new button somewhere in the screen will be added to allow players to select any card from the entire card pool which will add the unit into the ghost deck.
Then, allowing players to summon the creatures they've selected into the battlefield.
To save developing time you could take the original deck system and remove the conditions which interact with the rest of the game outside the forge.