Can active employee contribute unused vacation time to 403(b)?

An active employee participating in a 403(b)(9) church plan wants to defer his unused vacation time to the Plan.

I know there is a provision (Rev. Rul. 2009-31) that allows for active employees to have their unused PTO contributed to a 401(k), but I did not find such a provision for 403(b)s. The only thing I could find in respect to unused PTO being contributed to a 403(b) is via a post-severance contribution (Pub. 4482).

Does anyone know if a 403(b) plan could allow for an active employee to contribute (either via deferral or arrangement with employer for a non-elective contribution) unused vacation and sick pay to the 403(b) Plan?

Look at the plan document.  The definition of Compensation and what is included or excluded.