Gmail: how to integrate another email

How to integrate an existing email from the same Gmail account into a new email?

I am aware that I can use To the front but in my case, I have a threaded email that already contains a conversation and I would like to have another email embedded while keeping the conversation in the thread. In addition, you can not transfer more than one email in the same message.

All the solutions that appear on Google concern the export of the email to a PDF file, then the attachment of this file, which is obviously not the same thing, because I wish to be able to copy a existing email and then paste it into another identical email. So, I can paste image or text content preserving HTML formatting, HTML headers and metadata, that is, in a similar way to .EML files on desktop mail clients. .

I've tried to drag the e-mail I need into the Compose form, but this has no effect, even if the e-mail is movable, but apparently only to access a specific folder, and do not to integrate in another email.

Apparently Google has no support for embedded emails yet, or did I forget something?

What would be the workaround for keeping embedded email as formatted text with headers and all metadata? (Except To the front which would break the thread of conversation in my case)