can cmiCopyDocumentFromAppian be called from a script task?...

Yes, you should be able to. One thing to note is that if you want to use the local-credential, you need to activity chain the script task to a user input task. In this case, the document will be uploaded as the person who completed the task (or if you activity chain to the start event, it will be the person who initiated the process)
The other option is to run it with site-wide credential.

Yes, you should be able to. One thing to note is that if you want to use the local-credential, you need to activity chain the script task to a user input task. In this case, the document will be uploaded as the person who completed the task (or if you activity chain to the start event, it will be the person who initiated the process)
The other option is to run it with site-wide credential.