I don't remember many details so you have to wait for more competent replies but afaik there are 2 ways of getting extensions working on UGC: download the extension file using some workarounds and install it manually, or install a special proprietary module that enables Chrome extension store so you can use it like on Chrome.
Agree with a lot of the comments here, whether it's privacy or not wanting to use GAFAMmmy tools, there are better options than ungoogled chromium. For privacy: https://privacytests.org/ (maintainer works at Brave, but by and large this site is useful)