If it is possible (which I doubt), the only way to do so is if you put your Mac Pro into Target Disc Mode but even if you do that, I don't know if you will see all HDD's in your Mac Pro or just the main HDD with OS X on it. The whole point of Time Machine is to separate your backup data from the/a main computer so that if you're computer fails, your data is protected. With your method, it kind of defeats the whole purpose. Why don't you just get another external HDD and make that as a backup or take out the HDD in your Mac Pro, put it in an enclosure and make it an external HDD?