Hello @user004324 ,
The way our system works, HTML isn't currently accessible or editable in our regular email builder for legal and compliance reasons. Since you're looking to capture HTML that's already put together, our custom code editor also wouldn't be of use for you.
The closest thing, based on your request, would be to become a partner. Have separate accounts for your clients, and have yourself set up as the master account they're connected to. With your master account you'll be able to construct emails in our builder, then put those into your clients' accounts for them to fill in their specific content.