How can I use an external hard drive as a secondary internal hard drive on OS X
The only way to do this is to move your users home folder over to the firewire drive.

see: http://chris.pirillo.com/how-to-move-the-home-folder-in-os-x-and-why/

Of course, this also means the firewire drive will now always have to remain connected, and you will lose some speed due to the fact you are interfacing with firewire and not sata.