https://github.com/VSCodium/vscodium/blob/master/DOCS.md#how-to-use-a-different-extension-gallery
You can use the official extension repo with vscodium. You just have to edit the editors file accordingly. I dont have the link right now, but it should be easy to find.
https://github.com/VSCodium/vscodium/blob/master/DOCS.md#how-to-use-a-different-extension-gallery