Into "the" lounge?
In Fra they will usually swipe your card, so even a valid card must have a functioning magnetic strip.
If you are very desperate, you get someone's Amex Plat (usa one), and you photoshop a A4 sized webcheckin boarding pass, which matches the name on the card.
You go with this to the 'common' lounge, where priority pass is accepted.
(If a bar code on it, make sure its creased enough so that it won't be readable, even if it were a real one)