SSIDs are not a locally override field available.
What might be feasible depending on your needs is create a new SSID in the template and use SSID availability & device tags to only broadcast this new SSID to the site & AP(s) you need.
Example
- In a template bound to 20 networks create a new SSID called "Seattle"
- In the Seattle network tag the APs with a name like "Seattle" or whatever you want
- In the template under SSID availability select the Seattle SSID and choose "Only enable on APs with any of the following tags:" and select the tag you created
Result is this SSID will only show up on the Seattle APs and no other sites.