It really isn't necessary to do this, you count off 5 cards, their card was in those 5, if you perform it properly there should be no reason for them to suspect anything has happened.
Although, if you really want to do this, the easiest way would be to use a duplicate, count the cards and then steal their card into a gamblers cop.
Without duplicates, you could maybe show the cards, palm off their card then take the deck off them and replace it and pass it to the centre.
As I said, this really isn't necessary, in all the times i've performed the biddle trick there's been no suspicion at all.