Two key advantages of Whenjobs over cron are a simpler syntax for writing rules and a powerful dependency system that lets one job depend on variables set when other jobs run (allowing, for example, one job to run only when another job has finished successfully).
Instead you must use this style:
whenjobs --set variable=value
You may need quoting if the value contains a space. If you are using the optional --type parameter, put it before the variable=value declaration.