I'm new to MJML and I'm wondering if it's possible to convert HTML to MJML? I've tried searching for answers but the results are always the opposite.
No, it is not possible.
I guess your question is, "Is it possible to convert HTML to MJML automatically? Is there any tool for that?"
That is not possible. As far as I know, there is no such tool.
You can only manually "convert" HTML to MJML. Basically, that means you write MJML code that produces specified HTML output.