Extensions are just HTML, JS, CSS, and any media assets you may need. Anything that needs browser extensions/libraries isn’t supported, nor is anything that requires use of eval or similar.
For legal reasons that may be problematic, and not a very suitable business model even if you could get this off the ground. You should contact legal@twitch.tv as regardless of if what you want to do may technically be possible they may not allow you to do this for legal reasons.