Purchased additional leave

I have set it up for the employee to partially cash out PTO hours.  Some companies allow this for long term employees, that have more time than they can use.  They cash it out and reduce the employers liability.

Is that what you are asking for?  Or are you asking for money from the employee to purchase additional vacation time?  If so I would think to set it up as an employee deduction type.  Then manually create an adjustment to the PTO Bank.