Changed UOM in Req and PO

Hi, If the UOM of item is different than the UOM of the supplier then how will we change the UOM of the item in the purchase order to the UOM given by the supplier.
Which functionality will i use so that when i run auto create pruchase order the purchase order sholud have the UOM given by the supplies ??
e.g.
The item defined is in KG and the supplier will supply it in Drums
such that 1 Drum = 100 KG i have made the UOM conversions for it.
 Thanks