MohammadAG, above, has the answer for you. All of the sms messages are stored in the eventlogger database on the phone. There's nothing to stop you writing your own messages to the same db, which can then show up in conversations as any other message would. In basic terms, all you need is a simple app to gather your text message into a text field and use a SQL statement to write it to the correct eventlogger database tables. Perhaps even simpler, you could execute the SQL statements containing whatever fake info you'd like to the database directly from xterm.