@ MLP See? I fitted a Poser ball prop over a Poser box prop. (both from Poser primitives.) using default settings.

The fitting room does NOT require support files.

You can fit whatever you can throw in Poser over whatever else you can throw in Poser .

Figure over prop, prop over prop, cloth over prop or figure, the room does not care at all.

When you can open it in Poser you can fit it to whatever else you can load into the app.

More examples all over here : see page 5-6
Also for layered clothing.
But certainly read my Green Post on page 2 and the post with the limitaions (pink)

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!