The scenario is simple; what you want is to configure people search and allow that to search using a custom metadata property that is being populated into the user profiles from an external system like a BCS system. I will dedicate several posts for configuring the user profiles service and synchronization from an external source. But for this post I assume that the user profiles synchronization is up and running and that we added an extra profile property for example employee ID from an external system. So here are the steps.
So now your users are able to search for a user profile using their HR system employee ID.
Please note one thing if you want to localize the name of the property you will have to change the field name in this line to whatever you want.
<Property Name="EmployeeID" ManagedName="EmployeeID" ProfileURI="urn:schemas-microsoft-com:sharepoint:portal:profile:EmployeeID"/>