Hello
Centrify Connectors for Express or when used for AD Proxy do not currently have a way to set the scope of lookups, other than a registry key that can be added to specify which DC's are used for lookups.
I would suggest adding the Idea to limit the scope of a Connector(s) to our Idea Exchange located here.
https://community.centrify.com/t5/Centrify-Idea-Exchange/idb-p/Centrify-Idea-Exchange
I hope this helps!
RyanV