You create an ISiteRestriction object by calling the ISiteRestrictions.Add method, as in the following example, where objSite is the name you give to the ISiteRestriction object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSites, objSite
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog2")
Set objSites = objCatalog.objSites
Set objSite = objSites.Add "Microsoft.com", False
...
'Release objects
Set objSite = Nothing
Set objSites = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing