@sam
I'm afraid we have no such feature. The only way to do this currently is to export your database, edit it to add an activity, then import it again. So, practically impossible for most people.
But there is always our issue tracker where you can request new features. Not sure if you're the same person, but this specific feature was requested there just yesterday: https://codeberg.org/Freeyourgadget/Gadgetbridge/issues/5416