Is there a way to base a yearly accrual on the plan enrollment date?

When Expats are put on assignment they will get 8 days of 'home leave' each year that they are on assignment. But in this case, the 'year' is based off of when the person became an expat/was enrolled in the plan. Is there a way to do this without creating a custom Seniority Date and using a FF to grab that seniority date?