Hi
I would be surprised if a shifter exist that one can use on the brompton for all 6 gears directly. You CAN change the 3gear-shifter for a grip-shifter (Sram or Sturmey Archer) but then you still have the 2gear-shifter.

Hmhm actually the brompton solution is something like dualdrive (which I agree is odd for so little number of gears) but perhaps the SRAM Dualdrive is a possible starting point for a home-made modification. Tell us when you were lucky
Somnatash