The API does not have any support for modifying VS objects right now.
For non-VSX clusters, you would use set-simple-cluster. You need to provide the whole cluster object including all interfaces together. Any interfaces not in your list get removed from the object. You want .interfaces' "List: Object" parameter form. You will probably want to set security-zone to true and security-zone-settings.specific-zone to the UUID of the zone you're trying to set.
For single firewalls not in a cluster, it's basically the same, but the call is set-simple-gateway.
Separately, I urge you to reconsider this. Security zones give you a lot of ways to shoot yourself in the foot really impressively. They cause the same traffic to behave differently depending on which interface it arrives at the firewall. Using them is a mistake, and adding them to a policy which doesn't use them today is a bad idea.