Is there any way to walk slowly with controller or is it only possible with keyboard?

I managed to walk slowly and sneak on the marksmen and basic zombies in the early area of the game, you have to push the left analog halfway instead of holding it all the way. It's just not that precise and a bit tricky, but it's doable.
If you can calibrate the deadzone on your left analog, try to lower it for more leeway.

Unless you mean walk even slower and then I don't know, sorry.