Adding outlet to 4-way light switch circuit
Yes, that's the only way to do it. You need an always hot and a neutral and the fixture is the only place where you can get them. You would pigtail the black and white  wire at the yellow wire nut and pigtail the white neutral before it connects to the light. Are you sure there's not another circuit or outlet that you could feed your new outlet from? Also, the white wire at the fixture and into the first box from the fixture should be taped with any color but green, or white, to indicate an always hot feed. Black is a good color since most people have black electrical tape. It's also a good idea to tape the travelers with yellow tape for easy identification in the future.