Yes, it is absolutely possibly for an American to get a pro contract in Europe. There are a few players who do, such as Ngwenya but you need remember he is incredibly fast, so he has something different to offer. You have to be extremely good, as there is already a lot of competition back in Europe, and the Rugby over there is better than in America. But if you train hard and give your full focus, and are able to offer something slightly different, then if you play well you can get scouted.
Actually a good chunk of the USA Eagles squad are playing professionally in Europe. lf you look at the Wikipedia entry for the Eagles ( } you can see the current squad's club sides. At least a dozen are playing with French, English or Celtic League clubs. The best known is probably winger Takudzwa Ngwenyang-- raised in Texas despite the odd name -- and currently starting with Biarritz.