You can absolutely add your own html to the Source Code in the editable section of a marketing piece. To do this (in the add general content step of the template creation process), please click on the editable area. When the tool bar appears, click Tools > Source Code. A box will appear full of beautiful symbols and you can replace your html with the html that is already there. Or, you can edit the existing html. Click Update.
Note: Not all code is created equal. CSS will become different, and the content should be reviewed. The styles could be lost, and those should be reviewed and edited.