Hops taste bitter, whether they are isomerized or not. Eat a pellet and you'll know. So you may not add any IBU's doing FWH with no boil, but if you add hops you are adding bitterness
You won't need much for a Berliner anyway. Just off the top of my head, I'd add 1oz of a Magnum (or something neutral) to the FWH, should be good
-Ton