We don't have an extension but we do have a bookmarklet that lets you split or playlist any YouTube or SoundCloud track from participating websites.* You can find the Split it! button in your username dropdown.
If the website is constantly posting new music then you can create a Source from any of your favourite music blogs, subReddits, YouTube channels and SoundCloud profiles by clicking the Sources tab on your profile. Then any tracks added to those sites will be available to listen to in your Stream.
* The bookmarklet won't work on Facebook.