Welllll...that might be tough. I would suggest against it. I believe the stock G20 speakers are 2Ohms and usually aftermarket ones are 4 or 8 ohms.
Also the stock amps act as a crossover to keep the low bass frequencies out so you dont blow the tweet. You would need to install some type of crossover.
If you mis-match speaker brands and such they can sound shitty too. Really nice speakers are matched and have very little if no tolerances between the left and right sides.
I would suggest buying a componet system for the front that has 4.5's for the doors and new tweets. It should come with a crossover and it will sound great and make your life easier. I have eclipse componets in the front and have the tweets mounted in the stock position. They sound great, and loud.
If you really wanna try to do it (and probably blow them)...you can pull the A-pillar cosmetic covers off and just run a wire down after soldering it into the stock speakers...good luck.
Get rid of the stock tweeters if you are going to put after market speakers. From my experience, it ruins the sounds of your decent speakers. If you want to play it loud, the factory tweets overpowering the good speakers and it's very annoying hearing the tweets begging you to lower the volume. Just rewire your speakers so that the factory tweets will be disable.