One person cannot own more than one AdSense account. Hence if you have a YouTube channel and it has completed all the monetization criteria, either you have to create a new AdSense account or have to link with an existing one.
There is no separate AdSense for YouTube channels and blogs or websites. If you got the AdSense account from your blog, you can easily link the same with your YouTube channel when it gets monetized. Likewise, you can add a new blog or website on any of the previously approved AdSense accounts. Even though you have an existing AdSense account, your channel or blog should go through the strict review process by Google AdSense Team.
YouTube has given the facility to change the AdSense accounts linked to a particular channel. The unlinking or changing of the AdSense account is possible through the AdSense association page accessible through the SEE MORE link under the Video Advertising option in the Monetization tab in the YouTube studio.
