Hello,
Why would you like to open a .DAT policy file?
Opening the file manually probably would corrupt the file as these files are Encrypted by Symantec.
Infact, you can import the same .dat file in the SEPM to check the contents.
How to export/import an existing Symantec Endpoint Protection policy
http://www.symantec.com/docs/TECH106486
Again, check try these...
How To Open a DAT File:
The easiest way to open a DAT file is to double-click on it and let your PC decide which default application should open the file. If no program opens the DAT file then you probably don't have an application installed that can view and/or edit DAT files.
Tip: Use Notepad or another text editor to open the DAT file. Many files are text-only files meaning no matter the file extension, a text editor may be able to properly display the file's contents. Depending on where the DAT file came from, it could be a generic text-based file so this trick is worth a try.
Hope that helps!!