Can an employee substitute unpaid leave during FMLA with paid leave?

FMLA leave is not a paid leave category and defaults as leave without pay. An employee may elect to substitute his/her annual leave and/or sick leave, consistent with current laws and OPM's regulations for using annual and sick leave, for unpaid leave under FMLA. The employee may also be eligible to use leave earned from the Voluntary Leave Transfer Program, Leave Bank Program, advanced annual leave, and/or advanced sick leave. An employee may not retroactively substitute paid time off for leave without pay previously taken under the FMLA.