You can use any Micro USB charger, it just wont charge it as fast as the factory PlayBook charger.
this is due to the supplied mili amps of the charger, more mili amps means faster charge.
Most devices limit the amount of mili amps and will not draw more regardless of the supply given.
The lithium batteries also have protection circuts to prevent over heating and over charging.
read more here
Charging Lithium-ion – Battery University