Look at the link Randy posted in post #3 of this thread for your answer....
But, aside from possibly running the risk of blowing up your scanner, a scanner uses a "dumb" charging method based on a timer. In other words it does sense or know when the batteries are full, it will continue to charge them for a set amount of time no matter what. This over time will cause over-charging or if you elect to go with a higer mAh type battery to prolong your run times, it can result in under-charging.
Either way, using the scanner to charge your batteries will eventually degrade them and you will find yourself having to buy AA NiMH battery packs often.
In sum, charging your batteries in a proper separate charger will keep your scanner safe, keep your batteries conditioned and will save you a lot of grief and money in the long run.
Just my 2 1/2 cents.