I'm trying to make a staff that just adds a blanket +2 to spell attack damage rolls cast by any attack spell from a PC. For example, the Firebolt cantrip's (with a magic attack roll) damage increases from 2d10 to 2d10+2. And the Fireball spell's (spell save DC) damage increases from 8d6 to 8d6+2. I thought Staff of the magi would be a good cheat sheet to work from, but it doesn't appear to be what I'm looking for.  Does the Homebrew tool have a magic damage modifier that increases spell attack damage rolls?

No.

And if it would work on Fireball, then it wouldn’t be “Spell Attack Damage Rolls” since Fireball is not an Attack spell. (It requires a Svae, not an Attack Roll.) What you mean to say is “adds a +2 modifier to Spell Damage.”

Thanks for the help! So; specifically adding a +2 modifier to spell damage isn't something built into the Homebrew creation tool. Is that considered too game-breaking? Or does it just not make any sense to have one magic item that increases all spell damage, regardless of the spell cast?

The homebrew tools are only designed to be able to replicate the same effects that official stuff can do. The only spell damages that official stuff increases are Eldritch Blast from the Agonizing Blast Eldritch Invocation, and Cleric Cantrips from the Cleric’s Potent Spellcasting subclass feature.