While creating a new site, we will get an option to enter the device Serial Number manually if the device is not discovered automatically. Likewise, If there is an AP connected and UP on the same site, we will not get an option to enter the device serial number manually. The only way to add the new device is through the automatic discovery process.
If we want to add more APs to the same site, it should be part of the same subnet. If you move an AP from the existing site to a different subnet it will act as a sperate master irrespective of its being part of the same site. This scenario is called as "Split-site".
This is not a supported design and not recommended by Aruba as many features will not work if APs are part of the same site but on a different subnet.