Don't add any salt when you put the beans in the pot -- add it at the table. I don't have a slow cooker so I don't know if salt will harden the beans in a slow cooker -- it will in a pressure cooker, which is what I use -- I just add the salt at the end. I don't think salt is a problem when cooking beans in a regular pot so maybe it's okay in a slow cooker too.
Sorry I don't have a better answer for you.
By the way NEVER EVER cook dried kidney beans in a slow cooker. They are toxic and can actually cause very severe illness. You should only eat dried kidney beans that have been cooked in a pressure cooker or boiled at a hard boil for 10 full minutes which never happens in a slow cooker. You don't mention what kind of beans you are using so I just wanted to mention this. If you're not familiar with the issue you can read about it here: en.wikipedia.org/wiki/Phytohaemagglutinin<
/a> It's why I don't even own a slow cooker, I make beans all the time and use a pressure cooker always.