It looks like that there is no way to do this. If you take a look at tip for
Content only page type you will see the following ('Suitable for pages whose only task is to hold content, for example, individual news articles on MVC sites. The pages don’t contain presentation properties and tabs. This allows content editors to use the pages without unnecessary distractions.
Note that you cannot convert 'Content only' page type to a standard page type later.').
So you can try to think of creating new page type. And writing some custom code(task) that will copy content from the old page type to new one (using Kentico API).