How to integrating Yahoo Publihser Inventories with 3rd party ad server?

I want to integrate Yahoo Ad Server with 3rd party ad server. below is the scenario

1. Publisher A has web sites in Yahoo Ad Network
2. Publisher A has some unutilized or underutilized inventory in Yahoo
3. Publisher A want to add these inventory details in another 3rd party ad network say XYZ
4. Now Publisher A want a script (Ad Tag or Ad Code) such that
a) ad request goes to XYZ Ad Server
b) XYZ ad server serves ad for this Publisher A ad zone/space

What is the procedure for acheiving above flow?