Settings | Notification Server | Resource and Data Class Settings | Resource Associations | Asset’s Status
You could use a combination of checking if it has a status, as it doesn't when first created, even though it shows a "Active"
Then if it does break the association
http://localhost/altiris/nswebservice/resourcemodel.asmx?op=BreakAssociation
Finally create one
http://localhost/altiris/asdk.ns/ResourceManagementService.asmx?op=CreateResourceAssociation
How are you planning on calling the WS?