You can charge via USB but I don't know how well 'balanced' it is. Take a look at puremist. You can get 2 Samsung 25r and a 2 bay nitecore charger for £25 with free delivery
Not had the k200 but I've owned a few kanger mods and found the usb charging to be slow.
I'd say it's best to have an external charger and that also allows you to have a spare set of batteries if you don't want to be without a charged set.