How do I add a message to a conversation in Outlook

I use Conversations View in Outlook mail. Occasionally, a responder in the conversation replies with a message that Outlook does not recognize as part of the conversation. It appears o.k., i.e., the subject is in the form, “RE:…” but I suspect his mailer is doing something that Outlook can’t recognize. Is there something that I can advise this person or can I add his message to the conversation?