I think that the only 2 ways to entice an American driver to compete in Formula 1 are:
Either find one that is already competing in junior series on F1 tracks, or get a youngster and put him/her into such a series
or: Commit to at least a two-year obligation. To expect a driver to learn both a new type of car and a bunch of tracks they have never seen before and be competetive all in the first year,as was the case with Michael Andretti, is farcical, particulary with testing and therefore seat time, being limited by regulations.