Not so straight forward, if you want it to sound good you will need to match the sensitivity of the footwell and tweeter and I am not sure if that data is available. If the speakers you fit are lower in sensitivity the mid range may appear attenuated, if they are higher in sensitivity (most likely) your bass may appear attenuated.
Mind you regardless, it is still likely to sound better than your current setup with no mid range speaker at all!
A better sounding option is to go for a three way set up and replace all your front speaker. I used Focal 165 AS3. But bear in mind it will be a fair amount of work to do a good job of the installation. As someone else posted sound deaden the panels while you are at it, then when you close your car doors they won't sound like a Mk 1 Fiat Uno.