If you're travelling F with LH on departure from FRA, you can go to the First Class Terminal after arrival. You're effectively not going to be in transit though - you have to go through passport control (i.e. effectively enter Germany) though before you can get to it, and then you go tthrough immigration and security again - although this is of course a somewhat faster and more pleasant experience in the FCT. Actually, the lounge matrons in the Senator and Business lounges pro-actively recommend F passengers to go to the FCT rather than stay in the air-side lounges because of the much better facilities and experience - remember that from the FCT you get driven directly to your next flight in a car.
Andy