In Linux you can do it from the shell. I assume since OSX is just BSD (with the security ripped out) which is fairly close to Linux you can do it there.
taskset -c 0-3 /whatever/7daystodie.dmg
Something along those lines. If you want to set it AFTER 7 Days is running, use the PID.
taskset -cp 0-3 12345
Where 12345 is the PID for the executable.