A.) Slowly add new ingredients, ideas or projects. Add little by little. “Work” refers to adding something to a mix and to work in is to add it in small amounts. When making gravy, for example,  you add a little flour to roux rather than pouring it all in at one time creating a lumpy mix. “I’m teaching her the English language but I work it in during our evening conversations.”

B.) Work in can also refer to what career or business one is employed. “I work in finance.”