If you want to import your own html template and use it for your your email template or for your landing page template, then you can simply follow these steps:
Create a campaign or landing page
Select any template then click on the drag drop editor
Click on reset button
4. Drag an drop the "html" block
5. Click on the html block and paste your "html code" there
6. Click on "Save now"
Once done you can view your imported template in desktop/mobile.