I don't know whether or not this can be done, but I would check to see if a mt. bike frame can be converted to a 29er. That's all a 29er is, really-- a mountain bike frame with 700c wheels. Well, that's not exactly true, because the purpose-built 29ers have slightly different geometries, but it's true enough. So I would look into either buying a purpose-built 29er, or modifying a mt. bike frame. I suspect that "all you have to do" is adjust the brakes location. Somebody else will know more about it than I do... Maybe on the Mt. bike forum?