I don't know of a way to do this other than adding a separate "manual" activity with your best estimates of time, distance, etc. Even though it's an out and back, the stats of your out effort could vary from the stats of your return effort, so just doubling the activity wouldn't be accurate info.

you would have to use another file(old) and split it then time shift all the times. Simply record a new activity from the start and to where you started to that missing point , there is site that has time adjuster & file combiner function https://www.fitfiletools.com/#/top you could then first adjust the time back and then merge that file with your file

That makes sense. Thanks, Maybenaught.