@Greg Keys unfortunately I think you are correct that
ExecuteScript is the best way to achieve this right now. As far as I know, the
PutFile processor cannot append to an existing file. You are given the option to deal with conflicting files using "replace", "ignore", or "fail" as a resolution strategy. You should submit an Apache Jira to add this functionality. I could see difficulties with file locks and flushing the buffer given the streaming nature of NiFi and I think further investigation is needed.