To my knowledge, the only way to change or add to the default Kindle screensavers is to install and implement the jailbreak and screensaver hacks.
For whatever reason, the developers of the Kindle OS did not include a way for the end user to do this natively; this has been true since the Kindle 1 and thus far Amazon has ignored all requests from users to include this capability in the OS. Why?? Who knows - maybe they are just control freaks, or maybe they think their customers are too ignorant to be able add their own screensavers without generating another million "how to" requests for customer service!
As stated by FF2, the hacks are very easy to install and use, no programming knowledge is needed as all of the work has already been done by the hack author. In my case, I used the tried and true hacks programmed by NiLuJe. No damage was done to my Kindle, and the "anti-hack" police haven't shown up at my door.
If you aren't comfortable using the hacks, then you will have to wait for Amazon to implement "self-service" screensaver choices in some future version of their OS - and you may have a very long wait indeed.