Unzip the files you wish to be a pack.
Take all the files out of add-ons and put them all into one folder.
Rename all but one of the server.cs files to something else.
In the one server.cs you keep, put at the top of it
exec("./WHATEVER_YOU_NAMED_THE_FILES.cs");
Do that for each of the renamed server.cs files.
Make sure that it's proper packaged(folder, server.cs, description.txt).
You should be good to go. You don't need to zip it unless you share it with other people.