Adding poles to motor
Hm, no.

That motor was designed for exactly the shape of magnet used. You'll practically have no chance replacing that with something stronger. 

Then you'll change the cabling, i.e. you'll rip out all the hard-lacquered copper and wind something new. Which requires you to get a different cutout to hold your coils. At that point, you're not replacing something in the motor, you're building a new motor from scratch. Without dedicated machinery or experience!

You'll definitely have no chance of replacing the magnet with something cheaper than just buying an electrical machine that gives you closer to what you want, or an appropriate DC/DC converter.