Yes, you can!  (Exceedance Notifications, on the other hand, are for identifying when you have over x number of events that match a certain criteria over a specified number of days/weeks.)

In the Config App -> Type Maintenance, on the far right column of the Auto Assigned Lists -> Notifications, you see the column “Severity”.  Click on this label (it will be None, Custom or All), then click on the [ … ] icon that appears to the right of the label to open the Edit Severity window.

Note that the severity can be triggered on Actual or Potential, and pay special attention to the “Include events when Severity is Not Specified” (there is one of these on each of the Actual and Potential Severity tabs), as this controls what should happen if the users don’t specify an Actual (or Potential) Severity.