Francesca, there are definitely some terrestrial species you could have outside year-round, like Cypripediums and Bletillas; you might also be able to grow some Pleiones.
Where in NC are you? I'm in what used to be zone 5b/6a (7a on the new map
) and this past winter was so mild (basically an 8b winter) that I had a few tropical epiphytes outside most of the time, only bringing them in when temps dipped below freezing overnight.
Microclimates around your house may enable you to grow a surprising number of things that wouldn't otherwise be hardy, for example a sunny south-facing brick or stone wall can radiate back enough heat at night to raise local temps 5-10° F. Leeward (usually downhill) sides of buildings can offer frost-protection as frost condensation seems to flow downhill as it forms. Some species like Laelia anceps, Dendrobium moniliforme, Neofinetia falcata, and Vanda coerulea are from climates that normally experience cold winters, so they and their hybrids may be able to survive light freezes outside in appropriate spots. I'd speculate you could find some good matches for your climate (highs and lows) in mid-altitude Brazilian and Mexican species as well as plants native to S. Australia.
Really the only way to know for sure is to experiment a little with plants that you don't mind damaging/losing. Plants that you get now and grow well outside all summer with gradual introduction to colder temps will probably fare better than something introduced suddenly to the cold. Most orchids do better if dry when cold, though there may be some exceptions like high-altitude Pleurothallids, Dendrobiums, and Bulbophyllums, none of which would probably like your outdoor summer temps. Many hardy plants that will survive extreme cold under a snow blanket will die if unprotected or if hit by ice or frost, so winter precipitation is important too.
Another user, epiphyte78/Carlos
, has experimented a lot with naturalizing orchids in Southern California and has put up some great posts detailing his experiences with this. Checking out his old posts may give you some good leads. I hope this helps and good luck!