No problem! Adding tweeters is very easy. Just follow template and cut out holes for mounting in plastic pieces in window corner. Or just drill small hole for speaker wire and surface mount tweeter. I have done this on a few cars and it is fool proof. Buy good component system with 6.5" woofer and tweeter and it will have plans for you. You could also put new full range speaker in door that handles more power and add tweeter and run them off the same amp. Look at
http://www.crutchfield.com for ideas and then
http://www.smilephotovideo.com to get good prices. Good stereo store will have mounting kit to swap new head unit in for crummy stock unit. I like Blaupunkt Casablanca. It fills hole by adding a junk compartment under stereo. Then add an amplifier by mounting it under one of the front seats. It is an easy upgrade and will sound great for $400-$800. have fun!