This article explains the reason why the "Restricting client IP address" feature is only available in LWCS and not in IWCS in OfficeScan.
The feature was not added in IWCS because of a possible performance issue in the OfficeScan server where it is attached to.
If there are more OfficeScan clients (>1000) that will be using Smart Scan, then it is not reasonable to enable the integrated SPN server because of performance perspective. In this scenario, you should set up a stand-alone SPN server and edit the approved/blocked list there.
On the other hand, if less clients (<1000) will be using Smart Scan, then enabling the Integrated WVS is enough. In this case, you can use the approved/blocked list in the OfficeScan Web Reputation Service configuration section, which can support the granularity to domain/client level.