Obviously, the Alan Pangborn we know who was played by Scott Glenn was killed at the end of Season 1, so, unless there are more transdimensional breaks the only option for Glenn to reprise his role as the sheriff would be a flashback to when Pangborn and Ace had their issues.
The important thing to note here is the history Ace has with Alan Pangborn, who was one of the central characters in Castle Rock Season 1.
Obviously, neither of these possibilities have been confirmed right now, so we’ll have to wait to see what happens.