The .dat file output by FDMEE is not meant to be edited. Doing this via event script sidesteps the "Quality" part of Financial Data Quality Management, Enterprise Edition.
I would use the open interface adapter (or universal data adapter if you are on 18.104.22.168.100+) to load records from TDATASEG.
Regardless any decision you take, the reason you see those characters is because the .DAT file generated for HFM is not UTF-8.
In order to append, you need to use codecs module to open the .dat file and then append.
You should start by opening the original dat file with notepad++ and see its encoding :-)