Welcome.
For the -file option, you'd need the actual password value in the file and would thus not be able to use the environment variable. User-defined Project level environment variables created using the Administrator are only instantiated into a job's environment when a job runs, so would not be available in your shell script before the job starts.
If you are doing something else and you're certain that $DSPasswd exists in the shell before you issue the dsjob command, post your syntax.