The reorder information is actually held in a seperate table called SupplierList.
This should do the job.
INSERT INTO SupplierList (Item.ItemID, Item.SupplierID, Item.ReorderNumber) SELECT Item.ID as ItemID, Item.SupplierID, Item.SubDescription1 as ReorderNumber FROM Item WHERE (((Item.SupplierID) Is Not Null And Item.SubDescription1 '';
Once your happy that the above worked the way you wanted it too, run the following query to clean up the SubDescription1 field.
UPDATE Item SET Item.SubDescription1 = '' WHERE Item.SubDescription1 ''