LIke you said, you need an editor like Map Editor to edit and add objects to the existing map.
There are very clear, concise and detailed installation steps on the original gta5-mods.com page of Map Editor.
That is all the information you should need to edit the map.
To use it in FiveM, I suggest following the Map Resource part of this topic [How-to] Install a FiveM resource / Script
Good luck