How to add wysiwyg (like CKeditor) to Disqus module textarea?
Unfortunately, this is impossible. The Disqus comment form is in an iframe originating from disqus.com. It is basically a separate web page embedded within the content of your website.

If your page were to access the Disqus form, it would violate the Same-origin policy. This policy is to keep your information safe. Imagine if another page could read or modify data when you log in to your bank. Bad news.