We do not offer this functionality. Allowing forwarding of messages is not ideal because not all e-mail templates look good on every e-mail client/device. Additionally, most messages that get forwarded lose some of their formatting. As soon as new HTML gets introduced into the e-mail template bad things can happen when the message is rendered in a different e-mail client.
Your best bet would be to copy the content of the e-mail into a plain text editor to get rid of the HTML formatting and then paste that into the SendPress editor.
I suppose one other thing you could try would be to forward the message to WordPress, which can be configured to receive e-mail messages and convert them into posts. Then you could import that post into a newsletter and send it out.
If you want to completely replace the HTML with your own you can follow this knowledge base article.