Does the bike have gear levers on the frame down tube? if it does, you can still find 70's ear;y 80's type brake levers that have additional levers fixed to the inside of the hood by a pivot screw, these additioan levers then bend in and back around the inside of the bars and look similar to the small levers you see on cross bikes as posted above. Sorry but i don't know the proper name for these but I'm sure a good bike shop may be able to advise. If this works on your bike it will be a much cheaper and easier option than adding additional in-line levers.
Hope this helps