We have made a CI custom code table called criticality with High Medium Low. This is purely used as a guide to support staff so that they get a better idea of what they might be dealing with when handling a CI or Service Call/System Incident related to a CI.
But I am not sure about setting priority from the CI, as a critical CI could have a low impact, non urgent -> low priority system incident (eg server CPU usage high for a short time). So the priority should be driven by the nature of the sevice call / incident.
You could define a custom field for the priority of each CI and include that on the CI form. Then, if you made the text of the CI code the same as your priority text, you could probably pass that text in a "Data Update" type of DB rule to set your Priority to the Priority of the CI.
If that didn't map across cleanly, you could use a "Data Update from External System" where you pass the ID to a web-API script which would use the ID, lookup the CI, evaluate the priority of the CI, and then set the priority of the call appropriately. This option is more work, but remains an option if the above does not work for you.