No. There are two BWR wheels. The 6spd will only take two cogs secured with a snap ring,the 12spd will take 4 secured with a lockring. It was switching to the lockring that allowed the space for extra cogs. I don't know of anyone selling just the cassette body,and it might require other mods as well to get it to work.

FYI,The 2/6spd triangle is called the Mk 5,the 4/12spd is the Mk 6 or Advance. The Mk 6 has different dropouts,a different derailleur mount,different rack mounts(the rack no longer mounts to the brake),and a straight bar to mount the rollers which is stronger and separate from the main frame tubes.