You create an ISitePath object by calling the ISitePaths.Add method, as in the following example, where objPath is the name you give to the ISitePath object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSites, objSite, objPaths
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.Item("Microsoft.com")
Set objPaths = objSite.objPaths
Set Path = objPaths.Add "C:\Tmp", False
...
'Release objects
Set Path = Nothing
Set objPaths = Nothing
Set objSite = Nothing
Set objSites = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing