That's not possible under CGC's witnessing method. Basically the item has to go from the signer directly to CGC, and if it's done at a convention only a CGC authorized witness can take that item from the signer back to be graded. The person with the forged item would be still holding their forged item, as there is no chance to swap it out.
That said, I don't know that it's necessary in the sports card world considering how many cards are pre-signed and authenticated by the manufacturers. That doesn't exist in the comics world.