Hi,
the best way would be if you export your requirements using reqif to your contractor and your contractor links the requirements and then send them back also using reqif. Reqif will take care about all the ID issues as it uses internal ids that are mapped on both sides.
Of course that will have you to setup the new process but I think it is worth it.
How did you do in the past? Csv, excel, doors archives?
Regards,
Nikolai