The necromancer "command undead" ability isn't registering my character's +2 arcane grimoire, and is calculating the DC for the ability based on my spell save -- without the bonus. 
I was going to just overide the glitch by adjusting the DC, but I can only adjust the "to hit" modifier through customizations. Any tips? 

I'm unsure if this is intentional based on RAW. The arcane grimoire states:

"While you are holding this leather-bound book, you can use it as a spellcasting focus for your wizard spells, and you gain a +x bonus to spell attack rolls and to the saving throw DCs of your wizard spells."

However, since this is an ability, it might not have gotten lumped in with the rest. 

At the same time, the command undead ability states:

"That creature must make a Charisma saving throw against your wizard spell save DC. "

If my dm and I decide that we want this to include my bonus from the arcane grimoire, is there a way to adjust this?

dndbeyond doesn't support situational bonuses - and the magic item only works while you are holding it (which is a situational bonus).