Having gained "create tag" privilege I noticed tags are created when asking a new question. I can't seem to find how to add a new tag to an existing question. Is this possible ?
Create tags abilities come at 300 rep. This allows you to create a new tag. You can add existing tags to questions through suggested edits.
It's in the reputation section of the FAQ. I notice you taking charge of touchpad questions (thanks for that) - feel free to ping me if you want to retag some questions in the meantime.