Recurring assignments can be extended in bulk from the Extend recurring assignments tool.
Steps in this task
Selecting a new extension date that's before the date a recurring assignment is already extended through will not change the end date on that recurring assignment. For example in the screen above, choosing a date of 12/31/16 would not change any recurring assignments already extended through 1/1/2017.
After using this tool, run the Index update tool - it will help connect newly-extended job slots to existing job assignments.
There's a scheduled task available to do this for you on an automated basis.