You can merge two datasets in Excel by copying the contents of one dataset and pasting it at the end of the other. You need to make sure the columns of both datasets are in the same order before pasting.
In Dataiku, you can use the Stack recipe to append one or more datasets to another. The recipe automatically lines up columns with matching names across the datasets. Columns without a match may be kept or dropped from the output dataset.
For more information on stacking datasets in Dataiku, please consult the reference documentation.