Identifying an asset as critical is typically done by either having an attribute that idetnifyies the feature as critical, or it is known that certain pieces of equipment/asset types (system valves, pumps, etc) are critical to the network infrastructure.
Adding a field to the model and populating it during data conversion is a good approach. If you needed to take this attribute to the next level and use this attribute for some more advanced forms of analysis like identifying critical valves to perform isolation or counting the number of critical devices in a pressure zone, you could also configure a network attribute for critical assets and assign this field from your various classes to be a network attribute.