If trying to add custom words to Office 2016 in Outlook, but it is grayed out:
In Outlook, select “File“, then choose “Options“.
Select “Mail” in the left pane.
Select “Spelling and Autocorrect…” button.
Select “Proofing“.
Select the “Custom Dictionaries…” button.
Set “CUSTOM.DIC” as the default.
Ensure the “Dictionary Language” is set to the language you wish to use. Make sure it is the same as the “System Default Language“.
Add to Dictionary should now be available
