Allow the extension to have Remote Call forwarding through system programming, #322-XX-1
Program the remote number into one of the extension's personal speed dial locations (80 through 99), Feature 00, Feature XX, Telephone Number, Feature 00
Add a Forward button to the extension, Feature 00, Press a Black Button with a Light, Feature 11-XX-99, Feature 00
Press the Forward button, the light comes on. Any calls transferred to the extension will cause the system to pick any of the lines that are available to that extension, dial the number in the speed dial location, and then conference the lines together.
Press the button again, the light will go off, and calls will go to the extension again as normal.