am i able to attach tri bars to my existing hand bars that are curved downwards? or do i have to buy a base bar which have handles slightly going in an upwards directiont to attach the bars to ??

Yes your tri bars should fit on to standard road bars without a problem. you just need to check that the diameter of the bars is the same as the tri bar attachment clips.

You will be able to adjust the rake of the tri bars once they are on. But, one warning!
Don't be a doofus like me, and make sure that you don't crush and clamp the gear/break cables! Easy to do, expensive to fix!