Formal:
* Appending: Used when adding something to the end of something else.
* Augmenting: Used when increasing the size or quantity of something.
* Supplementing: Used when adding something to something else to make it more complete.
* Combining: Used when joining two or more things together.
* Conjoining: Used when joining two or more things together, often in a way that emphasizes their connection.
* Amalgamating: Used when merging two or more things together into a single unit.
Informal:
* Putting together: A more casual way to say "adding."
* Throwing in: Used when adding something casually or without much effort.
* Sticking on: Used when adding something to the surface of something else.
* Adding up: Used when calculating a total by adding several numbers together.
* Summing: Used when calculating a total by adding several numbers together.
The best word to use will depend on the specific context and the desired tone.