sebastien bourdais had a successful Carrer in Champ cars before coming to F1, thats the first one that springs to mind. A few other drivers spent a couple of years in Champ cars before moving to F1. Perhaps not a long Carrer, but as many has Schumi and Co, in the Old Sportscars champ, Montoya, Villenurve. Mario Andretti spent years in america in variuos Cups before adding to F1 to the list.
Theres proberly loads more too, most from further back in histroy though has nowadays the only real way to F1 seems to be through the feeder series. and also lets face it, if youre 26 now looking for an F1 drive, apart from a drive at the back of the grid. your F1 carrer is finished before its started.