Severity is an attribute of the Alarm and not of the Device, so I don't see how would work for OOB with the current Event Configuration. I think ackjo04's suggestion is what you're looking for.
In addition to that, you can create your own Alarm display filters to show only devices that ar important to you - for instance devices that are in a Global Collection, where you can add them manually and decide which device is highly critical to you. Once you have them in a GC, you can create a SANM filter and have different notification profiles for them.
All of this can be done via configuration, without any customizaton or extensions.