How do I add electric option to my water heater?

Depends on WHICH Atwood Model.....
versions 9e and previous (6 gal) and 3e/previous (10 gal) use separate controls for gas and electric

Versions 10e and 4e use common 12VDC for controls of both with a Relay to trigger 120VAC to element

So Really matters WHICH model you have......data sticker in outside compartment will show model number
Post it......OK