Sounds like a useful feature - you can add it as a feature request here: https://ivantiitsm.uservoice.com/forums/904372-service-manager
Feel free to comment with the link here so people can upvote it
As a workaround, you could duplicate the field, have one editable and one not editable, then apply visibility expressions to show the relevant control to certain roles.
Drop Down (Editable) -> Visible: $(CurrentUserRole() in ("Admin", "ProblemManager"))
Drop Down (Non-Editable) -> Visible: $(CurrentUserRole() not in ("Admin", "ProblemManager"))
I would probably show both for Admins, but give it a try and see if it works for your use case