Hi Ivan,
You can use pub.utils:generateUUID to generate the UDDI key. After generating the UDDI key you can set it to the headers/query etc before sending to native service by using the Invoke IS Service policy of API Gateway. You can refer Invoke webMethods IS policy in API Gateway 10.2 for how to achieve this.
And yes, you need designer to do this.
Regards,
Vallab