We need to create a custom attribute in Active Directory that will contains lookup values and the user object will need to choose one or multiple values in this lookup. The lookup values will be defined in this custom attribute. Our application will be able to read this custom attribute.
User is related to a Campus.
And just one campus can be selected. The list of campus is managed in the Active Directory.
How we can acheive this by extending the schema?
You should really NOT do this. Extending the schema to store something like that is like using a nuclear bomb to open a packet of cheezeits.
Active Directory has plenty of attributes that never get used, in this case I suggest the location attribute (l), division or department.