That is easy to do, the wire is under the sill plate on the floor then goes up the door jam and out to the door.
But you really need to consider the speakers you are using. If you keep the 2 way (tweeter and woofer combo) in the door, and you tap the same wire at the sill plate, and run that to an extra tweeter for the A panel then you are running a 3 or 4 ohm tweeter on top of the 3 or 4 ohm 2 way on the same channel. This is additional resistance for the channel and you will cut down volume to all of those speakers substantially. Also having 2 tweeters that close together with fight each other and not sound good.
You really need a component speaker. Get a component that comes with a tweeter, the 6x9 woofer and crossover.
You can then tap the door speaker wire at the door sill plate and run that into the crossover. Mount that under the dash. Then you can use the existing wire going to the door, hook that to the woofer output of the crossover and that connects to the door speaker using the stock wire. Then you run a new wire off the tweeter terminal of the crossover up to the new a pillar tweeter.
This way you reuse the wire going through the door, have the a pillar tweeter, and proper signal and resistance on the channel.