normally, to boot from another volume, you'd plug in an external drive with OS X installed, then in System Preferences->Startup Disk you'd select your external drive, then reboot
But if you don't have an external drive to boot from, you could boot your MBP into target disk mode (reboot while holding down the "T" key)
once you've done that, you'll see the FireWire icon moving around on your screen. At that point, you can plug a FireWire connector into your MBP and connect the other end to another Mac. Your MBP's internal hard drive will then mount on the desktop of the 2nd Mac as if it were just an external FireWire drive
I've not used any defrag software on the Mac, but I assume that from there you could run the app on the 2nd Mac with your MBP's FireWire-mounted drive as the defrag target.
when you're done, unmount your MBP's drive (on the 2nd Mac, select your MBP drive, then Command-E), then unplug the FW cable. Then, reboot your MBP to get it out of target disk mode.