What you are talking about are called inline levers, or sometimes cross levers.

Here`s a guide to installing them.

http://www.parktool.com/repair/readhowto.asp?id=146

They work really well if set up properly. Not to be confused with the old-type of under the bar levers that were integral parts of older brake sets called "suicide levers".

I put them on my Surly LHT because I found it useful to be able to brake easily in a heads-up postion for commuting.