I understand how back up files are used to restore your old game if you delete your extension pack, but I don't understand...

I'm getting tired of starting my sim family again, I want to continue playing with my sims I have now. After 2 extension packs, I learned that it deletes your saved game when you add a new extension pack. And Recently I bought sims2 seasons and I really want to install it! but I want to play it with my old game file! I don't want to start a new family again.
is this what installing back up files do?
So lets say a family called Johnson lives in my sim town, when I install my seasons, and use a back up file- will the johnson family still be there? and would they be living in the seasons world?

Is it possible to still keep my old sims in a new extension pack? if this is not the way to do it, is there a way?

Yes, if you back up your My Docs/Sims2 folder somewhere safe then after you install a new EP or SP if there is a problem with your game then you can put that folder back in to restore your families. But your families shouldn't be getting deleted when you install an EP or SP. That usually only happens when you uninstall EPs or SPs. This faq will explain everything that you should do before installing an EP or SP. http://www.modthesims2.com/wiki.php..._Next_Expansion

Yes, that's weird, I've never heard of your families being deleted when you install a new expansion pack!! You should double check it.