then Under Admin->Custom Destination you can add one new custom destination. the "Custom Destination" field will be "to-external-transfer,s,1" (without quotes)
Then, you would probably set up a an incomming or outgoing route, or maybe a custom trunk which has its destination set to the one you created. (I'm really not sure with FreePBX)
Sorry I can't be much more help.. I haven't got any spare hardware to play with. but I'll let a more qualified person chime in :P Eventually your dialplan can look something like this
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