Setting Recurring off days

Greetings all,
We have implemented an 80-in-9 policy at work; that is we work 80 hours over nine days and have every other Friday off. It is not staggeringly obvious how I might reflect this in the calendar settings for OmniPlan. One way I might do it is shorten the normal work week to four days by nine hours and add Extra Hours on every other Friday.
Is there a way of making Extra Hours recur without creating them individually? I do have a multi-year project in the works and that would be 26 special days for each year.
Thank you in advance for any help.