I think that there should be support on the new Steam release for the old saves. I got pretty far into the game, and I just want to continue from where I left off!
Since the game is on what is essentially a new engine, you will not be able to port your saves to the new game. Besides, you’d be missing a lot of stuff if you start this from the middle. Start from the beginning and see the new stuff! 
You might be able to use the sv_unlockedchapters command in the console.
We are not physically able to support old save games. When the game binaries change in a certain way, save games break because they rely on old binaries. This is impossible to work around.