It's possible there is an American-to-Eccentric dealy, but you may not get full adjustment (probably enough for any gear if you use a half-link). But if you're making a tandem, why not just space the BBs to fit the link chain; all even numbered 'rings would need a whole or half inch (18", 21.5" etc), all odd 'rings need a quarter or three quarters (19.25", 20.75" etc).

There was a lovely Jackson TT tandem in my LBS the other day, HED trispokes, DA 9spd (I know), Cinelli Angels, all black but the tubs (yellow) and bar tape (white). Mmmm, tandem-tastic