I have changed your wording in italic
We always use the plural for the phrase "in italics."
The sentence above is ambiguous, because it can mean that somebody has changed some text that was already in italics.
I have added italics for the changes that I made.
I don't think the verb "add" is the best option. I think I'd use:
- I've marked the changes in italics.
- I've used italics to mark the changes.
There is also the verb italicize.