The only way I know you could add some semblance of a signature is to make a shortcut. Then, whenever you want to post a signature, then all you have to do it type the two or three letters you've set as the shortcut key.
Note that the shortcut does not do word wrap/carriage returns, but that's a simple fix you can do once the main signature is in the post box.
Sorry, but that is all I can think of to do.
Marilyn