I have two questions about damage;
1) Can I add Pain Resistance (the Adept Power) and High Pain Tolerance (the Positive Quality) to damage resistance tests? And by this I mean the damage resistance tests after one gets hit in combat. I'm fairly certain the answer is no, but I just want to find out.
2) I vaguely remember reading that if a character takes damage equal to his/her body the character is stunned. And if any character, no matter the body, takes 7 damage from a single hit he/she is stunned. Is this true?

1. No. Both of these options reduce the effects of damage, not the damage itself. They reduce the amount of damage sustained only for the purposes of calculating your wound modifiers. Thus, if you have Pain Resistance 3 and you suffer 5 boxes of damage (total), then you calculate your wound modifiers based on 5 - 3 = 2 boxes meaning that you wouldn't have any modifiers (yet).
   
   
2. I think that you're thinking of the knockdown rules in the first case. If you take (stun or physical) damage equal to or greater than your Body, you're knocked down. If you're getting shot by Gel rounds, the threshold for a knockdown is reduced by 2. There is 'ware and adept powers that help mitigate this (foot anchor and rooting come to mind). Knockdown rules in full can be found on page 151 of SR4 and continue onto 152.
   
   As for the other one, there is an optional rule in Augmentation (p. 121) that indicates taking 7 or more boxes of (stun or physical) damage they may receive a more grievous wounding than otherwise. The book provides a list of potential wounds like blood loss, brain damage, broken bones, implant damage, organ damage, etc. While an optional rule, it's one that I personally use because it adds flavor to games and gives another reason for a Street Doc contact other than 'ware upgrdes.