If you're dead set on using a charger you'll need to do something to dampen the ripple and stabilize the voltage.
Without opening the charger and doing a bunch of calculations to figure out exactly what to put inside; the easy way is to run the motor off of a battery that is being constantly charged by an off-the-shelf battery charger.
But I agree that you will likely spend more time and effort than just biting the bullet and buying a proper power supply.
BTW, don't forget about the inrush current required start the motor when you're figuring out how many amps you are going to need.
