When proofreading students' essays, sometimes I need to add a comment between two words to suggest somthing is missing here. But it seems comments have to be attached to a word or words.

Can I add a comment between two words ?

For example, a sentence below:

He late.

I would like to add a comment between "He" and "late", and write "is" in the comment, suggesting "is" should be written in this place.