Purchasing Category Assignment

 Hi , I have a buyer i want to Limit and assign buyer with three purchasing category . So that the Buyer can only make PO of its assigned Purchasing category .
When on the buyer form PO category is assigned to buyer but only one category can be assigned but when i remove the purchasing category the Buyer has rights to all PO CATEGORY .
How will i limit and assign multiple PO CATEGORY to a buyer.
Thanks