chrt can not only change the scheduling priority of a running process (you'll actually need the pid) but also launch a new process following a particular scheduling policy :
chrt [options] priority command argument ...
chrt [options] -p [priority] PID
chrt sets or retrieves the real-time scheduling attributes of an
existing PID, or runs command with the given attributes.
So if you want to launch tar running SCHED_RR rtprio 10 straight from the beginning, simply
chrt -r 10 tar blahblahraroptions
If, as written in comments, it would make no sense to have all running processes running the same scheduling policy at the same priority level, there is just no way to achieve that.
It is however possible to launch all your users' processes automatically to your desired scheduling policy / priority since this is (per default) inherited by child processes.
1/ fork a bash process scheduled SCHED_FIFO / priority 10 :
$ chrt -f 10 bash
2/ Whatever process launched from now on will be scheduled identically without the need to use chrt.
/proc/2074 $ ps -eHo pid,ppid,tid,class,rtprio,ni,pri,comm
4168 2074 4168 FF 10 - 50 bash
4192 4168 4192 FF 10 - 50 ps
Use with caution and only for your test purposes, and of course do avoid launching fork bombs as well as assigning the highest possible priority !