I am trying to compose a feat that will add an ability modifier to the damage caused by all spells that cause a certain type of damage.  In this case, charisma modifer to all neucrotic damage spells.  How do I write the mods so Beyond D&D will add the damage during game play?

It I currently not possible to do that.

Thanks.  I will check later if this ever changes.

The only way it will likely ever become possible is if WotC publishes something official that does the same thing. Then, it still might take a looonng time before DDB gets it programmed to work. (DDB only got the Disciple of Life feature working earlier this year. The Life Domain Cleric is a PHB Subclass, and the PHB was published about 7 years ago.)