How can I add data to an existing dataset using an Dataiku DSS API?

Recipes are meant to have a different input and output however this can be bypassed. See this post for a sample:

Having said that this would require re-reading the whole dataset so it's not the most efficient way of doing it. Certain dataset technologies like traditional databases allow for data to be appended. In that case you can set the recipe to use the Append Mode. See sample below for a Python recipe writting to PostgreSQL dataset:

The sample code in my post uses the Dataiku API. It's Python code. If this is not enough then please explain your requirement clearly and what exactly you are trying to achieve.