I don't think there is a conversion tool for Hmail. Certainly not one in the normal place (http://portal.smartertools.com/kb/c180/utilities-and-conversion-tools.aspx). Depending on the amount of data / mailboxes you might have to complete them manually and individually by creating the empty mailbox the going into the user webmail and use the Migration tool (Settings, My Settings, Advanced Settings, Mailbox Migration)