For the State University Retirement System (SURS), things are calculated in a couple ways:
What earnings will be used to calculate my pension under the Traditional or Portable plan?
SURS uses a figure called "Final Average Earnings" to calculate your retirement annuity if you are on either the Traditional or Portable Plan. SURS calculates these in two ways and uses the higher earnings figure, either:
the average of the 4 highest-paid consecutive academic years of employment; or
the average of the last 48 months prior to termination of employment.
The academic year is the 12-month period starting on the first day of the fall term. For the University of Illinois, the period is August 16 through August 15. Overtime Pay and Summer Session earnings are included in the calculation. Accrued vacation earnings up to 56 workdays are included if they are paid when you terminate employment. Payment for sick leave is not included in the calculation. Also, with few exceptions, earnings during any academic year after June 30, 1997, which exceed the previous year earnings by more than 20% will be excluded from the calculation.
So to answer your question, it's entirely possible and regularly done.