Add in something—
1. put or pour something, especially in recipes.
- Continue to add in the flour and mix each addition until it is well incorporated.
2. attach or include something, usually in the middle of a calculation.
- Before you finish the totals, please add in these new figures.
Add on something—attach something at the end of a calculation, list, etc.
- She wrote me a cold letter, even the inquiry about my health looked as if it had been added on as an afterthought.