To include, mix, or combine something into something else. A noun or pronoun can be used between "add" and "in." I hope you added in the milk before putting the brownie mix in the oven. Can we add some more herbs in? The sauce is pretty bland. Ugh, I hate when they add patchouli in a perfume because it ends up overpowering everything else!
See also: add
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.