For this scenario, you should use QinQ tunneling:
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/dot1qtnl.html
This feature allows to receive all packets on an access vlan (tagged and untagged) and re-encapsulate them inside another vlan packet.
You have to check if your Switch/Linecard supports this feature and be carefull with the native vlans (client sent untagged by default)