Set user expiration Yosemite server.app How do you set local network user expiration using server.app? 

This action would block the user to log in after a given date.
In order to change a user's expiration, in the Server.app you need to go to "Users", then make sure that "Local Network Users" are chosen (using "All Users" keeps that option greyed out).  Then highlight that user in question and from the cogwheel button below the list choose "Edit Password Policy".

There you will find all the "missing options" like password complexity as well as expiration for various reasons.