If you aren't confident with your sculpting skills, an alternative is to use the Black Templar upgrade sprue's tabard torso fronts. You'll still need to apply some green stuff to fill the gaps between the power armour torso piece and the terminator torso piece, but it does work if you're careful with it. Obviously, it's not a full robe, but it conveys the idea and puts some "cloth" on the model all the same.
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