On the DM-620, can I rewind a file to a particular point so that I can insert, append or dictate over something I have just recorded?

No.  Unfortunately the rewind buttons will not work while recording a file using the DM-620. When you stop a file using a recorder in the DM series, it will immediately start a new file.  There is not an option to insert, append or record over data recorded. 

Note:  You can pause a file and then continue recording

Source:  In house testing

Updated 1/4/12