Set user expiration Yosemite server.app
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.