Add the possibility to ADD data to the existing (and not override all of them)

Aujourd'hui, chaque composant va transformer l'ensemble des données et donc proposer un nouveau jeu de donnée.
On souhaite avoir la possibilité d'ajouter facilement des données au dataset.

The issue names no files, tests, or entry points. Start by locating the component that currently replaces the dataset, then clarify the expected append behavior and dataset examples with maintainers. Done should be demonstrated by a test or documented example showing new data added while existing data remains.

Written by the indexing model from the issue text.