Watch the Various Chicks - Mid Strings online guitar lesson by Matthieu Brandt from Guitar Lab: Triads and Hendrixian Double Stops
In general you can add any of the notes that are part of the other triads in the key you're playing in to the triad that you're currently playing. By doing that you'll remain within the same harmony, because the sounds from those other triads will be connected to the triad you're on.
You're fishing for harmonies and melodies from the same pool of notes; the key you're playing in. This fresh look at your fretboard makes it easy to extend a triad. In this lesson we'll study the V chord and it's extensions.