how to setup different num sequence for purchase orders related to site is......plz help me

hello friends…i got one problem…

when i am raising po(purchase order) it generate a new num.when i am selecting the vendor…but my task is po num should come when i am selecting site id for po…AND it should be different for every site…

is it possible plz help me friends…thanks in advance

You would need to customise the system to achieve this. What is driving the requirement of a different number per site? The PO has the site on it so it is instantly recognisable to the site making a site number sequence redundant.