How to charge a Nikon d5000 camera while in use?

I'm using it for a time lapse (interval mode) and I don't think the battery will last for 999 shots. Can you charge it using a USB or something while in use?

You can't charge while in use.

You can use an A/C adapter (sold separately, of course) to power the camera indefinitely. There are also battery options like a Quantum Battery 1 with appropriate cable, which will give you a few thousand shots on its own.

Your best bet is two batteries. Charge one while using the other. Or, as suggested, a 110 adapter.