I don't believe you can customize the sig's at all. Your best bet is to make a custom dictionary word thing where typing something like sig* would change to your desired signature.
@McStoner - you can customize the signature to emails if you go into email setup and then into each email account seperately to do it.
I'm not sure if there is a setting to make sure they are applied to forwards or replies though.
(Note: usually the "sig" is default is set to say 'sent from my blackberry'....so this sig can be changed to whatever you want. You just have to go in and change it.)
on the main page click the menu button (BB button)
choose "Setup",
go to "Email Accounts".
choose "Internet Mail Account"
choose your email account and click the menu button and choose "edit"
under "General Options" go to signature and change it to what you need. i hope it works.