Blogger – Registering a GoDaddy Domain Through Blogger

This would require the assistance of Google to resolve. A search turned up articles mentioning this link: https://spreadsheets.google.com/viewform?key=pZHHZdeYKeHitQbvxkvwnCg&

If Google is still monitoring that form, you may be able to enter your domain name there to have them resolve it. If that doesn't work, you'll have to post on the Google Help site for Blogger and hope that someone from Google looks at/resolves your problem.

Most likely someone else used to own that domain name; now that you're the owner, they should be able to remove the old record to allow you to use it.

I'm not sure why you want to keep this content separately from the rest of the blog. Blogger has been supporting non-blog pages for several years now. If you want your content to be displayed on every page, you can add a widget with that content.

If you, after all, still want the content to be hosted externally, Blogger supports <iframe /> tag at least on non-blog pages. Here's tag reference from W3Schools.org.