Yes, it is possible to create an email content block without using MJML code. There are various ways to create email content blocks, such as using HTML code.
Many anti-spam/anti-virus software remove the html comments used by MJML in the body of emails, so emails received by recipients differ from the preview shown by creatio.
Could you please explain in more detail when and why end users have to use an external HTML editor, and what issues you are facing with the current editor you are using? Additionally, I'm interested in learning more about the problems with your markup, so that I can better understand your needs. If possible, please provide me with a detailed description of the situation so that I can provide you with more specific information.