Hello TuRuu,
If your cluster is built on only IAP-205 APs, you cannot automatically add the IAP-207 to the cluster. The IAP-207 needs to have the same software version as the IAP-205 APs before you add the AP to the cluster. The IAP-207 is using a different image (Vela) compared with the IAP-205 (Taurus).
The best way is to boot the IAP-207 stand-alone. Upgrade the IAP to the same software version as the IAP-205 cluster and add it to the cluster. That should do the trick.
In the future, you can add additional IAP-207 automatically. They will get their firmware pushed from the cluster. You can also update the firmware of both IAP's via the cluster.