You can do the smart prop thing without opening the Hierarchy editor.  Outside of Poser, you can create a folder where you want it to go in the prop library. Then, back in Poser, you just put the prop where you want it to be, open the folder, click the plus button, name the prop and it'll ask whether you want it to be a smart prop then.

That's the way I've found easiest

CHEERS!