Are you asking about adding Alt Text for an image using Microsoft Word? If so, it's a cinch, as it is under most HTML editors, too.
If you're referring to something after the fact then it becomes significantly more complicated and murkier.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore