Weber used to sell kits, but discontinued the practice. Most likely due to liability. Some people don't know that gas is dangerous or somehting like that..
You need the manifold or the gas valvles, the regualtor hose and any assorted hardware and gas fitting sealant made for gas. The burner tubes are the same. (At least on the old webers.)
You can find parts if know which ones you need. Weber won't sell them unless you own a gas grill with the correct serial number.
If you show us what you are working with, we might be able to help.
I see used or free natural gas grills on craigslist all the time, maybe he can find a parts donor.
It doesn't hurt to ask any of us here. Lots of closet gassers here..
Good luck with the project.
AJ