Yes.. the new Chaos codex doesnt care what you call your army or what you have in it so to speak.. RAW you could even make a Wold Eaters army all out of Nurgle/Death Guard marines and the only thing you would be violating is Khornes honor (if he has any).
Although really, they shouldn't be packing banners anyway... Icon of Chaos Glory at the most. Night Lords aren't exactly what I'd call devout. They're just a bunch of sickos who get a kick out of blowing stuff up and/or terrorism.
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