long adId = Long.parseLong("INSERT_AD_ID_HERE");
long campaignAdExtensionId = Long.parseLong("INSERT_CAMPAIGN_AD_EXTENSION_ID_HERE");
// Create ad extension override.
AdExtensionOverride adExtensionOverride = new AdExtensionOverride();
adExtensionOverride.setAdId(adId);
// Create ad extension using existing id.
AdExtension adExtension = new AdExtension();
adExtension.setId(campaignAdExtensionId);
adExtensionOverride.setAdExtension(adExtension);
// Create operations.
AdExtensionOverrideOperation operation = new AdExtensionOverrideOperation();
operation.setOperand(adExtensionOverride);
operation.setOperator(Operator.ADD);