Hey ShadowKT, thanks for the hint. It is indeed very similar and can replace this functionality almost completely! If I would have knew it earlier probably this would be the easier solution :D. There is only one feature which is not supported by the static templates: my script can filter the groups by the name and limit them, so basically no one can smuggle "hidden" units into the mission. But honestly speaking, I don't think this is important as the people making the flight plans are trustworthy enough.
So yes, basically its pretty much the same. I am thinking about offering this as web solution, so you can drag and drop your mission files into it - select the groups and than merge into one environment mission, than it would become pretty handy to have this script and than this could be synced with the server ... but I don't have the time right now.
My recommendation for people not familiar with Python-Programming (and having fun to use it):
-> Use the static templates from the Mission Editior as recommended by ShadowKT (thanks for the hint!)
I currently don't see any downside using the static templates.