Hi @ahuarte ,
Welcome to Microsoft Q&A forum and thanks for your query.
As of today it is not possible to implement custom user functions in Mapping Data flow. But sounds like a great idea and as per my discussion with internal team the product team is reviewing this feedback.
I would also request you to please share your feedback in ADF user voice forum: https://feedback.azure.com/forums/270578-azure-data-factory
All the feedback shared in this forum are actively monitored and reviewed by ADF engineering team.
Please do share the feedback link here once it is posted so that other users with similar idea can upvote and/or comment on your suggestion. Which would also help to increase the priority of feature implementation. Once we have the feedback link, we can take it forward to ADF PG to further review the feedback.
Hope this info helps.
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