WowAce

As a healer trying to switch from Grid, I am unable to find a "healing-reduced" type status.  I find it an important and useful status, and I'm wondering if there's a way to efficiently add it to Grid2

What Grid2 version and language of WoW are you using?
Grid2 r587  enUS

You can use Raid Debuffs for this purpose. For example in halfus you can see the healing reducing debuff  in your tanks and the number of stacks. Raid Debuffs has and Advanced configuration tab where you can enable or disable the debuffs you want to track (or add new debuffs).
Raid debuffs tracks automatically all important bosses debuffs. If you want to add some specific debuff not related to bosses, maybe for pvp, you can create  the debuffs from status->Debuffs. I think there are only 7 o 8 healing preventing debuffs that players can apply to other players.