Thanks for your answer.
The main goal is to import our HTMLs and then be able to create reusable fragments and make use of some features (customizable fragments, etc.).
So according to you, there is no way of creating an HTML that would leverage the email diesgner content components without needing to use the converter button (which may mess with layout or structure)?
I would still like to see some documentation on what is going on behind the scenes when this button is pressed, though.