in the Mail.app Signature preference pane, there is a pull down option "Select Signature". specify randomly (or sequentially) then create a bunch of new signatures.
it requires copying the quotes from a source file, but does what you want it to do. looking ~/Library/Mail/Signatures.plist (where the sigs are probaly stored) makes me think that there is no easy way to do what you want, as far as parsing them from a file into mail.app.
adam