Christoffer,
If the switches are all in the same group, you cannot have just one switch have a unique vlan. Said another way, if all switches have vlan 10 and 20, but you want one switch to also have vlan 30, an overide will not address the issue (sorry cappalli). That is an AMP limitation today for configuration of the Mobiliy Access Switches and Mobility Controllers. The assumption has always been that switches in the same group would only have minor variations in things like IPs, hostnames, etc, and not whole new elements. I would recommend submitting an idea portal entry for this.
However lets say you have one set of switches that need to have vlan 10 and 20 and another set of switches that need vlan 30 and 40 and they ARE in the same group, you CAN do that by using a bulk override. What you'll do is click on "Modify Per-Device Settings" on the VLAN itself and you can adjust what the VLAN ID will be across a variety of switches.
I hope this helps.
Best regards,
Madani