There's sadly no stats editor in the editor of DOS 1 (unlike DOS 2). You can get the stat files to edit either from:
Divinity Original Sin Enhanced Edition\Data\Editor\Templates\Stats\ (unless you have the GOG version of the editor, I guess), though I don't know if they're completely up to date,
or by unpacking the file Main.pak (to be found in the main directory of the game) with Norbyte's tool; s. this thread where you can find Norbyte's download link: http://forums.larian.com/ubbthreads.php?ubb=showflat&Number=572322&page=14
When you unpack the Main.pak you can see the folder structure for installing the stat files in the game's directory. If you created your own module with dependencies on Main and Shared, you should store the stat folders and files in your mod folder in: Divinity Original Sin Enhanced Edition\Data\Public\[YourModName]\ instead of in ...\Data\Public\Shared\ or ...\Data\Public\Main\ (though there are a few, like Requirements.txt, which must be stored in the subfolders of Shared to make them work for the main campaign; ItemCombos.txt [used for crafting recipes] can be stored in the mod folder but won't overwrite the main campaign's item combos; to overwrite, they must be stored in Shared).
If you want to edit stats of the main campaign in your module, just copy the relevant files over to your module and edit them. Otherwise create a new stats file (just a text file) with a new name.
You can learn more about working with stats by looking into mods like Baardvark's 'Scales' (https://www.nexusmods.com/divinityoriginalsin/mods/68
) or Ameranth's and Elric's 'Epic Encounters' (https://www.nexusmods.com/divinityoriginalsin/mods/103
) or some other mods. And there are some threads related to stats in the mod forum for DOS 1 EE (here).
I'm not an expert for stats and it's been more than a year that I did something stats related, so I can only give some information here and there.
If created correctly, the new stats entry should be selectable (in Stats) in the editor's sidebar (Ctrl + B) of your item's template or instance in the game world. Changes and additions to stats will only apply/show up after you restarted the editor/your module, in case you try to make changes/additions to stat files while your module is loaded in the editor.