Can I modify another object's value?

Hi experts,

     I extension purchaseorder, add a field ex_jhsl, my business is: modify the field when ConfirmedInboundDelivery, so I add code in event:beforesave of ConfirmedInboundDelivery object, now ,the field of purchaseorder object is read-only, did the studio support: can I modify another object's value when I am ConfirmedInboundDelivery object? Thanks.