There are some spam issues in the comments, so I had to disable comment editing for a short time. It'll be back once the spam bots are gone. FYI... they leave a comment with "great" and then edit in the spam stuff later, to get around the filter.
(Mainly, in context of industrial food production, and there, the noun "der Zusatz" is more common. NOT for normal cooking when you add salt or something.)
2.
to give a hard time
("jemandem zusetzen" - mainly for situations and circumstances that give someone a hard time. Think of continued stress at work or cold temperatures. NOT for a difficult task or a person giving you a hard time.)