Officially you can book tickets for passengers: humans. For pet and luggage you need to follow IR rules.
Yet if you want to have luxury of your luggage with you, you can take the risk of booking two tickets: one for your self, and another for some XYZ, and when TTE comes you tell s/he has just gone to bathroom, thus you save trouble of vacant berth allotted to other passenger, and also carry your 'dear luggage' with you:))
Norhing Official.