Are the outlets grounded?
First off, verify the two wires twisted together are really grounded and not just some addition that hasn't been completed. If they are grounds, then yes, connect a copper wire to the two twisted together, use appropriate wire nuts, and the other end to the ground screw on the outlet.

The wire the previous owner attached, is it in conduit with a metal box? if so, it might be grounded, you'd have to check that. The metal box you're installing will not be grounded unless you connect to conduit that is grounded or to cable that has a ground wire. Just going from a metal box to the ground on an outlet will not ground it.