So if orange detachment has 3 of warjack A printed in the base set, and then an expansion has one of warjack A printed in the orange detachment, you will be able to customize your orange detachment to have 4 of warjack A - three from the base set and one form the expansion.
This does not allow you to override the maximum of five of any one titled card though. So, if the expansion had 3 warjack A AND the base set had three warjack A, you'd still be limited to five (3 and 2 or 2 and 3).
The options for expanding and modifying detachments make this very intriguing to me. Also note, that there is nothing in the rules (or otherwise) to indicate that a warjack printed in an expansion (even the same titel) will have the same stats/ abilities as the one in the base set.