I remember that there are changes possible just like.. update existing missions to add extra units to make the mission easier to play but.. to make a mission by yourself is not so easy as it looks like, also there are many things to add just like triggers and teamtypes
i ever made many extra mission for C&C 1,. only iv tried to complete a own created mission but it doesn´t work so good. i think there is a lot of time needed to making it smoothly
And without creating linked options like triggers and teamtypes the own created mission
won´t work good so take the time to fix it.!