I understand I am replying late to this thread but I did find a solution that worked for me.
- Open the email you want to attach. Use the Forward as attachment option and send it to yourself.
- Open the reply you need to attach it to.
- Click the attachment button and select from Choose from files.
- Under Onedrive you should select Email attachments.
- You will find the email you want to attach in this list. Simply select it.