Fortunately there is a faster way to do this.
For this you obviously need editor and a map which has overgrowth set up that you want converted.
1. Backup
Backup the following files in map's folder:
- editor/gameplayobjects.con
- editor/layer.con
- ambientobjects.con
- staticobjects.con
- Gamemodes folder
If your map doesn't have an AI folder with an ai.ai file in it, copy it from a map that does.
2. Run editor
Run editor, load your map.
Go to Tools>Add-in Manager and load Singleplayer Editor plugin if not loaded already.
Place a combat zone counter-clockwise with Used by Pathfinding enabled.
Switch to Singleplayer Editor and click Generate Pathfinding.
This action will create a GTSData folder in your map (this is used for creating AI support for your map, but not doing that here) and more importantly, in the process it'll convert your overgrowth to statics.
Switch to Level editor and save and you should now have a new staticobjects.con with all your overgrowth in it.
Saving in Level editor will write/overwrite GPO and statics, that's why we backed them up.
You can now change or remove your overgrowth.
You can combine this new staticobjects.con with old one or load it separately or rename etc.