This article provides an example configuration for Q-in-Q tunneling on EX4200. The mapping option allows you to assign an S-VLAN to a specific C-VLAN on an interface.
According to the Juniper documentation, Understanding Q-in-Q Tunneling on EX Series Switches :
"The mapping a specific interface approach has two suboptions for treatment of traffic: swap and push. When traffic that is mapped to a specific interface is pushed, the packet retains its tag as it moves between the S-VLAN and C-VLAN and an additional VLAN tag is added to the packet. When traffic that is mapped to a specific interface is swapped, the incoming tag is replaced with a new VLAN tag, as previously described in How VLAN Translation Works. You must specify a VLAN tag with the swap operation; you cannot use the native keyword."
Example:
[edit vlans] user@Bridge-A# show v200 { vlan-id 200; interface { ge-0/0/10.0; ge-0/0/13.0 { mapping { 100 { push; } } } } dot1q-tunneling; }