In fact, even better: you can add forwarding to your existing connection. <newline>~C opens a command line, which accepts the following commands:

    ssh> help
    Commands:
      -L[bind_address:]port:host:hostport    Request local forward
      -R[bind_address:]port:host:hostport    Request remote forward
      -D[bind_address:]port                  Request dynamic forward
      -KR[bind_address:]port                 Cancel remote forward

(If you're not familiar with them, some of the other escape sequences are useful too. ~? lists them all.)

[EDIT] Apparently, if you have a recent enough version, you can add a forward to the master with `ssh -O forward ...` [1]

[1] http://serverfault.com/questions/237688/adding-port-forwardi...