From WarzoneWiki

How to convert old .wdg map files to the new .wz format

1. Create a folder called 'maps' in your Warzone 2100 GPL-main folder.
2. Use WDG-Explorer (Windows-only) to unpack the map into an empty folder.

   The folder contains the following files: *.gam-file, addon.lev, folder 'Map1' containing:

   DInit.bjo Feat.bjo Game.map Struct.bjo TagList.tag TTypes.ttp

3. Rename the folder 'Map1' into the name of the map, e.g. '8c-UltimatePower'.

   Do the same thing with the *.gam-file. (8c-UltimatePower.gam)

4. Rename the addon.lev to '[MAPNAME].addon.lev', e.g. '8c-UltimatePower.addon.lev'.
5. Create a new folder '/multiplay/maps' and move the folder of your map, e.g. '8c-UltimatePower', into that directory as well as the the renamed addon.lev.
6. Select the maps-folder containing your map and the renamed addon.lev and pack it into a zip-archive with '[MAPNAME].zip' (e.g. 8c-UltimatePower.zip).
7. Afterwards rename the file-extension to *.wz.

You now have the Warzone 2100 GPL-map-version of your *.wdg-map.

You can even add map-packages this way. Every map should have its own folder, but there should only be one *.addon.lev file, containing all the information about the maps.