You could say, "Can you salt the soup?" or "Can you add salt to the soup?" Salten, however, is not a word.

Can you add some salt to the soup, please?

I disagree with the comment 'salten' is not a word. I've used it and I've heard others use it. Perhaps it is dropped from usage in some countries (US ?), but still can be used here when you are talking about soups or broths. I've only heard or used it myself when talking about some liquid or liquid based preparation you're preparing in the kitchen - soups, etc. I would say, 'Can you salten it a little' or alternatively. 'Can you add a little salt'.