But you can use the 7th string IN a chord, if that's what you mean. They're notes, just integrate them into the chord. You can use the 7th string notes as root (bass) notes, or you can use them as inversions.
For instance. You can play an Emin chord, but throw in the low open B, and viola! You have an Emin 2nd inversion. (Assuming you're in standard tuning, with the 7th tuned to B.)
You can use the 7th string as the root as well, like I said. Play an Emaj form barre chord, using the 7th string, at say... the 1st fret, and bingo, you have a C major chord.