Can i Charge my Nikon D3100 while its filming?

I'm planning on filming my daughters school show and my battery won't last throughout the show
can i charge it via the usb port?

No. Your best bet would be to invest in a spare battery and change at the interval.

No!
And Nikon doesn't do a battery grip for that model.
You will have to break at a suitable point and swap batteries. Chances are the camera won't run that long any way - the sensor will overheat and shut the camera down long before the battery dies.

No, that camera does not have an on-board charger.

However, non-camcorder cameras that do video are limited to 29 minutes and 59 seconds by international treaty (how that happened is still a puzzle to me). And many cameras is less than that due to sensor overheating as described above. But even it the sensor didn't overheat, you are limited to under 30 minutes.

You can also buy an AC power adapter for your camera, but for some reason Nikon is very proud of it, so it costs over $100. A second battery is far cheaper than that solution.

Since you are limited for sure to under 30 minutes - just swap out batteries when the camera stops (but it will likely overheat and stop before then anyway).

If you absolutely need non-stop video, you must buy a camcorder with extended life battery. A still camera with video - regardless of model or brand - won't work.