I have an example function in the list monad.

twoSucc :: Int -> [Int]
twoSucc i = [i+1, i+2]

Now I want write something similar with the StateT monad transformer.

The simplest addition of StateT would look like this:

twoSucc :: StateT () [] Int

twoSucc i = lift [i+1, i+2]

twoSucc :: StateT Int [] ()
twoSucc = do
i <- get
put (i+1) -- how do I put [i+1,i+2] here?

In this transformation, you've moved your Int result from being the monadic result to being the stored state. I don't know which transformation you really want.

-Karl