The answer unfortunately is no.
A bit of background to understand why.
HTML is a mark-up language that does not enforce strict standards. When it comes to HTML email, this is particularly true as lack of support for modern HTML standards in many email clients is pushing email designers to use all kinds of tricks to try to get the message to render as they wish.
The lack of strictly enforced standards means that there are no recognizable patterns in the HTML code.
If standards are enforced, they are unfortunately enforced at the application level: your application may have applied specific HTML coding standards to your email design, but they may be very different from those chosen by another company.
The combination of the issues above - in turn - makes it virtually impossible for us to create a conversion tool that will read the code, understand it, and turn it into the structured JSON document used by the BEE editor.
The good news - although we understand that it is not ideal and not the answer you were looking for - is that creating new email templates with the BEE editor is easy and quick. We did so ourselves at MailUp when we introduced the BEE editor: there was no way for us to convert our own, "old" templates into BEE-ready messages, so we started from scratch and designed some new templates.
Now, if you still believe that the only solution for your company is to build a tool that will take your existing messages and convert them so that they can be used by the BEE editor (i.e. you will generate the JSON file required by the BEE editor), let us know and we will try to help, if we can. For example, we can answer questions about the JSON structured used by BEE.