progessing in chord theory?

now i know all the normative chords in every key but its when i add other notes to triads thats when it gets complicated for me because adding a 4th or fifth note to major or minor triads alters the original key your playing in. i.e say im forming a c major chord but then add a minor 2nd to it it changes th key from C to ? .

Well, you're actually making a C add b9 then. I'm not really sure what key to say that is in, but I would say it probably depends on how you're leaving the C add b9. You might use it to move chromatically to a related key or nearby chord.

Yes i thought that it did not belong to specified key maybe someone else can define it.? or maybe it is just a moving chord chromatic in the key of C i.e say C add b9 to a Dm or something thanks for the help foogered

Although I can't recall it off the topic of my head. A good way to figure out "scales" or modes for specific chords would be to look at all the intervals and find the scale to match it.

i.e. A good scale for D Dominant 7 would be a D Mixolydian because,
D7 has a P1 (D) M3 (F#) P5 (A) and m7 (C) which Mixolydian shares.

Well, I'd say for such a complex chord, it will probably hurt you more to make it so that it is absolutely in the key, but that's my opinion. :P

If you want to go about it that way, the notes in C add b9 are also contained in F Harmonic Minor as well as the following (somewhat useful) scales:

G Locrian 6
G# Ionian #5
A# Dorian #4
C Phrygian Major
C# Lydian #2
E Ultralocrian