No, you can use the subscription-manager client on a rhel6 system and subscribe it as noted in the instructions. or get the subscription certs from a rhel7 system and put them on the rhel6 box. the rhel6 box will need reposync installed.
Aren't the subscription certs for a RHEL7 box specific to that box that are generated by redhat after registering the server?
Is is as simple as moving those certs over to RHEL6 box and install reposync on it?
You use the subscription manager client to subscribe the box to rhn. That can be on rhel 5, 6 or 7. You can take those certs and use them from any box afaik. i have no idea if that violates any licensing from redhat though.
Why not register the rhel6 box ?
Will try this in Development environment.