It is not possible to change or add to the out-of-the-box priorities.
We've introduce a so-called label that addresses many use case we've seen where there was a need to change the default values for priorities. And these use cases specifically included service targets. And by the way, we've considerably updated the expression builder used during the service target definition in our 19.08 release.
As a starting point here's the doc on how to create your own labels: Adding custom selections to label or case source menus - Documentation for BMC Helix Business Workflows 19.08 - BMC Docu…
Product Management, BMC Helix Business Workflows