Hi ;
1- You can assign several purchasing organizations to one reference purchasing organization, but each individual purchasing organization should be assigned to just one reference purchasing organization.
2- You can create a contract with referance purchasing organization which is used all of purchasing organization if enterprise structure configurations related with -->Assign purch. organization to reference purch. organization be completed.
3-You have to maintain vendor master data for related company code.
Regards.
M.Ozgur Unal