Are you using a controller for the APs? If so, there is a way to set what is called an HESSID which would be the same MAC on all the APs for the BSSID (which is the SSID's MAC).
First let me know if you are using a controller before we dive into this.
In that case, using an HESSID, if the code is up to date should help you as it would make the BSSID the same MAC for all APs broadcasting that SSID in your network. The HESSID needs to be the MAC of one of the BSSIDs in your existing network. So, it has to be a real MAC from your equipment.
Try that and let us know if you have issues afterwards.
It's a bit more complicated than that, let's put it that way. It is a somewhat recent thing and not as easy as it sounds.
Aside from this, I do not know of a way to statically assign the BSSID to an SSID. I would recommend upgrading the code on the ZD to a newer version in order to use this feature.