You can export the activity for example in the TCX file format, edit the file in a text (or XML) editor, and import it back. Export also an existing Activity with laps, so that you see how it is done there.
I think that it sholud be the posibility to do in Garmin Connect... more easy.
Suggest it to Garmin: https://www.garmin.com/forms/ideas/, but I am afraid it is not a feature that they would put anywhere with a high priority onto their ToDo list. Manual editing of activities is not really their main focus, so it is a questionable whether you'll ever see such feature.
... there is another way to quickly see stats for a certain lap of the Activity - use the Trim function (see the cogwheel menu on the Activity page in Garmin Connect Web). It allows you quickly checking the time, pace, and some other data for any section of the activity. You can create laps both on the basis of the time and distance. The trimming is repeatable and reversible without any limits, so you can view as many laps as you want.
Just for the case you do not know it - the best way how to avoid the necessity to create laps manually in Garmin Connect, is setting up the watch so that it makes auto-laps for you, or adding laps manually during the activity by pressing the lap button (typically the button is disabled though, so you may need to activate it in the settings of the activity profile).
