You might wonder why you would make a new incoming message since those are messages that you receive from others. You might not think that it would even be possible. In fact, it's a way of importing messages into Microsoft Entourage. (The import command is just valid for the mbox file formats, which become message folders.)

You could read the content of an .eml message file in a way that is similar to reading a .vcf file, as described in Attach a vCard. Then you can use that as the source property, which is the only property you'll need, to make a new incoming message at any folder. Or, you can use it to make a duplicate message from properties of an existing message at a location where the duplicate command does not work.

The interesting thing is that this is possible with AppleScript, although not in the UI. Most of the "read-only" properties are settable at inception, so you can do almost whatever you want with the power of AppleScript. Just follow the same sort of procedure as making an outgoing message, but do it as an incoming message, to see for yourself.