According to your comment your goal is not to actually increase the load on the server, you only want to test the Nagios alert in the case of high load.
For doing this, I wouldn't try to actually increase the load of the server, the server load is hard to artificially control so this isn't a very reliable test scenario. Instead you could store the output of
sar into a file, edit the content of the file, and then use a the content of this file instead of calling the sar command to test your Nagios check.
sar 3 10 > /tmp/sar_output
vi /tmp/sar_output #write the numbers that you want
Then in your nagios check script, instead of calling
sar you just do
This way your test happens in controlled condition and is more reliable than by finding creative ways to increase your server's load.