When in the full screen view of outlook, and you have an email open, you can't drag an attachment, or another email message, into the message you are composing.

You have no trouble doing the the PC version of Outlook.

The only work around is to close full screen view, drag the message you are composing to one side of the screen and the main outlook window to the other side, then drag and drop whatever attachments or messages into the message you are composing.