No. The only way to do it after you've done the activity is to edit the activity data file (.fit, .tcx, ...) and upload that. There are tools that can do this.
Christian N, there are tools such asFit File Repair Tool, which is licensed software, that will do exactly what you need. If you like messing with fit files, it might be worth the investment. Otherwise, if you just have a few fit files that need laps inserted, you can zip and post them here. Someone might take care of that for you.
I'm not aware of a free online tool that will do this. Maybe someone will comment.
... as seilogramp wrote, use the FIT File Repair Tool if you want it comfortable. If you have some basic knowledge of XML (or even just HTML), you can export the activity in the TCX or GPX file format, and edit it manually with the help of any text or XML editor (such as Notepad, Notepad++, PSpad, etc...).
