Importing existing HTML pages
When you import an existing HTML page (not created by the Coherent Editor) it will opened in the Editor's code editor. Currently editing visually pages not created by the Coherent Editor is not supported.
Combing HTML pages created by the Coherent Editor with other HTML content
If you wish to combine content created in the Coherent Editor with other HTML content simply create the file in the Editor and place your custom HTML outside the <!-- Coherent Editor Start --> <!-- Coherent Editor End --> comments. However that would allow you to edit through the Editor only the HTML created in the Editor.