We have been asked some question around DataPreparation and security.
1. We do support impersonation given that you can specify the username on the input and output sides. I didn't fully get the group-related question and neither did @tfion... the answer is most likely "no", but can you provide more details?
2. Ditto above: we didn't exactly get the question.
3. It depends on where the source file comes from. You can do HDFS to HDFS (and per your "write back" below I assume it is what you're trying to achieve). But you cannot prepare an Excel file that you uploaded to Data Prep (or any "Local file" per the Data Prep terminology) and write it to HDFS. No specific security framework or policy to comply with.