I throw in the occaisional 7th - 7th chords tend to fit with standard major & minor fairly easily. Diatonic Harmony for Major chords and 7ths is the same except in 7ths the fifth scale degree is dominant, where in standard major it's major.

9th's are trickier as you'll find the added ninth often doesn't come from the key you're playing in, so it sounds wrong. Sometimes this can work anyway, but they almost always sound 'outside' to me.

If anyone has the secret on ninth's, I'd like to hear it too! :D

I've substituted add9 chords in standard progressions before, and that works well for a different sound...