I have searched for that as well. I don't think there is a way, though the admins may be able to speak with more certainty on that. It would be nice to add a line break for a comment in Smart Add.

Hi robhix,
This isn't possible but we appreciate the feedback! Depending on your needs using the email service might be a good option without adding too much friction. You can add a note there and it will add exactly as formatted.

Hope this helps a bit!

Here's a posted idea that you might want to vote for. There may be others.
https://www.rememberthemilk.com/forums/ideas/27087/