The only way to get something like a signature above the previous message's text is to edit accounts.ini (in the mail folder) and make the signature part of the reply header. That means the signature will be text-only (no HTML) but can be several lines by using character escapes (\n for newline, etc,). The file accounts.ini is just a text file (UTF-8 encoded but without BOM, if you need to include non-ASCII characters) ... other than that matter of encoding any text editor should be able to handle it. The reply header is on a line that starts "Reply=", there is a separate header for forwarded messages, and each account will have its own headers in that file. (If you want the recipient's mail client to recognize it as a signature you should start the signature with two hyphens and a space on a line by themself, that mark is what indicates the actual end of the message text.)

Well thanks for the tip. If you have could you give us a idiot proof step by step instructions?