You can in 2008 and up, and maybe in later service packs of 2005, but in any case you may not want to do that. In 2005, and in some scenarios still in 2008 and up, queries issued by SQL Agent against msdb bloat the cache significantly. The more often you run jobs, the worse the problem becomes.

http://connect.microsoft.com/SQLServer/feedback/details/526485/dm-exec-cached-plans-bloat

Instead of only scheduling the job to run every 20 seconds, maybe it would be better to have the proc that does the checking use WAITFOR DELAY inside a loop to check the condition every 20 seconds allowing the job to run continuously. The proc would only take action when the condition was detected and would always loop back to the WAITFOR DELAY.