There is not way to get Thunderbolt on a 2006-2012 Mac Pro. Thus, there is not way to use a TB monitor for those Macs. There is no TB dock for those Macs since to use a dTB dock the computer has to have TB
Also, you stated " If you had a mac with thunderbolt built in, you could use a miniDisplayPort to TB adapter,
No correct. There is no mdp>TB adopter, Although a mdp cable will plug into a TB port it only convays nmd (vide w/audio, it dose not convay TB dta. Yu can't use a mdp cable between tow TB device since a mdp cable is not interchangeable with w TB cable.