Maybe you could use a wrapper script to accomplish this?
If the given program always needs its niceness to be adjusted you could try the following:
- Rename the given program: mv given_program given_program.org
- Create a wrapper script named given_program
with the following content:
nice -n 19 given_program.org "$@"
Make sure that:
- The wrapper script is in the same location as the original,
- The wrapper script has the same owner/permissions as the original program.
BTW: The "$@" part makes sure that all the (possible) options/parameters are given to the original program.