No, you cannot do that.
UniWebView is a layer from system and is added above of the whole Unity view. That means it is always floating over your Unity scene. You can never put your elements from Unity above of the webpage. A possible workaround for it would be making your webview transparent, and put your Unity elements around/below the webview. But by doing so, these elements will not be clickable if it is under the webview (even they are visible).