I'm afraid in general it is not possible. Marking message as "forwarded" is not defined as an "IMAP system flag" in RFC 2060. Rebex IMAP component supports system flags only (they are accessible through the Rebex.Net.ImapMessageFlags enumeration).
Some IMAP servers may offer an additional (non RFC) flag for marking messages as forwarded, but we don't know any IMAP server that does so (e.g. GMail or MS Exchange 2010 support standard system flags only).
You can mark messages as "forwarded" for internal purpose of your application by using Custom flags (also called Keywords) if your IMAP server supports it. Working with Keywords is described in another article. But be aware that the keywords are defined by YOU - other applications accessing the same IMAP server will probably not support that keywords.