I know of people who have tried this. The short answer is that you *might* break something as you could potentially back-power the USB, but I have not heard of anyone actually breaking something. If your charger puts out a clean 5 V, in theory it could extend your battery life if you had enough watt-hours in your charger. If it is not a clean 5V, it could do bad things, and it might do bad things when its charge runs out.
Does your laptop manufacturer make a higher capacity battery? Or a replaceable battery?