The only real way to do this without expensive machinery would take two people and could spell disaster for your transmission if not done correctly. It would go something like this: disconnect the feed line at the radiator or external transmission cooler (whichever you are running) and run the line into a bucket. Have someone start the car while you continuously fill the transmission with new fluid from the top end. Of course at the same time, you would have to watch for the color change in the fluid coming out of the feed line. The tricky part is that you can only put so much fluid in the trans at a time via the dipstick hole and it may pump out faster than you can fill it. Plus, there's no way to gauge if you are keeping enough fluid in there during the process. The other person could just sit in the driver's seat and wait for you to start making an ass out of yourself and start screaming, "Cut it! Cut it!"
I'd never do it myself although I have done equally retarded things in the past to get stuff done. My advice is get a deep pan with a drain plug, or install one yourself in the stock pan. Then, just do several back to back drain and fills to get as much of the fluid changed out.