Thank you very much!

Are they able to wear a synthetic skin to appear as organic beings aka androids?

It may be possible, I suppose, it the outer covering is removed and some of the internals are reworked slightly. In the interest of practicality, though, I doubt this is done--being used by space-faring factions with limited resources and all, it probably doesn't make much sense to put artificial skin on them, when they typically already come equipped with a rather resilient outer casing that better protects their inner workings from the hazards of working (and generally existing) on board an interstellar cruiser.

What about having synthetic skin over the outer casing? They'd look a bit bulkier, but could still pose as organic people.

I suppose that's possible, too, although still not necessarily something they'd have much use for. To really make it convincing they'd have to rework the head to feature points of articulation in the jaw, eyes and ears, as I get the feeling that an unmoving synthetic skin face would be far creepier than a robot face. The most efficient way to make one more "android" might be for their creators to focus on making the head feature the necessary articulation, and make a good replica of the head and neck the mech can wear as a mask of sorts, and then have them wear some sort of space suit, armor suit or jumpsuit over the rest of their bodies. Of course, if they need one to appear organic in a pinch, they could just put it in a full spacesuit or flight suit with a pilot's helmet.

There might be some specific scenarios where that could could be useful, although I designed these with the intent of them filling roles more like those of the battle droids and/or protocol droids in Star Wars, than as infiltrators and assassins like the Terminators. It just comes down to different applications I suppose, but it is fun to speculate on possible modifications that can be made to them beyond what sort of tools or weapons could be added to increase their efficacy in their assigned roles.

One thing I like about these sort of "replica" robots over the more practical but "obviously a robot" designs is that these kinds can make use of equipment designed for their organic counterparts very easily--whether it's Sable, Didelphis Alpha, The Wraith, one of the Atlas Battle Units or a Mechanaut like this, they can be issued the exact same weapons as the organic soldiers (or even pick up weapons they find in the environment if they are intelligent enough), wear the same body armor if they need some extra protection, or hop in the cockpit of any vehicle they've been given the necessary programming to operate.