Originally Posted by JJeffrey

I doubt you'll have much luck. Most int'l carriers don't offer any kind of paid access to their lounges (via membership, day passes, etc.). Access is only granted via status or class of service.

I've been to the AF, BA, & LH lounges in TXL. All are completely average...perfectly fine places to kill an hour or 2, but I wouldn't want to spend all day there. If the Berlin Airport Club is independent, it may offer some sort of pay per entrance, which is probably your best (only) bet.