Is there a way to convert oracle data files from Simplified Chinese (HZ) encoding to Unicode encoding (UTF-8)

Assuming the NLS settings on both machines are correct and that the data in the source is encoded correctly, since you are just trying to migrate data from one database to another, then any Oracle tool should work. You can use the export and import utilities (classic versions or DataPump versions depending on the Oracle version). You can also use streams or materialized views to copy data from one database to another. Or you can set up a link to the database and query the data from the link of the database. The Oracle client automatically converts data from the source character set to the target character set.