Interesting thought!
Let me bore you with an explanation. I would assume they are outfitted in the same way we see the orcs dressing the Uruks in Isengard during a scene in FOTR. I think it could even be Lurtz rather than a generic Uruk. Obviously, trolls are a little more feral and mindless than Uruks, so the orcs would need to find some way to restrain them before suiting them up. Luckily, we also have visual evidence for that, in TTT when Sam and Frodo first come to the Black Gate, the trolls are whipped by orcs while they wrench open the huge doors to let the Easterlings pass through. So, with some restraint, the orcs would probably be able to fasten any buttons and buckles, possibly even apply a small amount of mascara before they unleash the beasts into Middle-earth. I'm going by the films because obviously its those that GW used to sculpt their miniatures.
I suppose you could even argue that the trolls might view themselves as equal to orcs, despite their seemingly brainless nature. In Moria, not once does the cave troll question whether he should be killing the goblins or the Fellowship, so clearly he is allied with the goblins. Therefore, the trolls might simply let the orcs dress them?
