Level 2: LEMMINGS
Level 3: SSENNIUG
Level 4: ACELRATE
Level 5: BFEYBEYB
Level 6: WALLIMAK
There's probably a code for Level 1 in the game too, but I don't believe how to extract / modify L3D's codes is known (let alone if Winterland and the full game handle it the same way).
On a side note, the RASPUTIN cheat also works in Winterland.