Paid days to volunteer

Hi Fabulous HR Gurus

We have implemented a new initiative for paid days to volunteer (3 a year), I've added a policy and procedure but do i need to add it to employment contracts? i can do a quick contract variation letter and we don't have a huge organisation so only mildly time-consuming with the help of mail merge but i felt like policy and procedure plus it adding to our employee benefits handbook section would be enough.

Any thoughts on the matter would be very much appreciated 

Michelle