Do we have provision for setting up header bidding tag prior to calling Ad Server ?

I want to build the final VMAP which can contain Ads of Direct as well as header bidding based programatic demand. So looking for following flow

1. Call Ad Server to obtain VMAP
2. Call header bidding and pass VMAP to it
3. Obtain extended VMAP (modified by header bidding Server)
4. Use extended VMAP