Can you tag someone in a message on Facebook?

You cannot “tag” people in messages per se, as the messaging function of Facebook acts more like emails than like the Facebook notes function. Once a message has been sent, the group of people reading and interacting with the message thread is locked. You cannot add to the group.

You can however, resend the message, with the same body, adding the other friends to the sender list.

So long as they are on your friends list it works the same way as @username or if you write a note you have the option of tagging people before you post the note.