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Doesn't appear so. When I try to change my house flag, the canon houses' flags are all 844/810 or something around that. The mod team made the house flags so they can only be used by the houses they are intended for. I'm sure with enough time and creativity you can get close to a canon flag, but it would be pretty weird having House Lannister use the Stark banner as their own.
 
I think you can via save editing, but it would be far simpler to just make yourself a member of the house via ruler designer.
 
You can add in a fake religion, give it the house flags with random=yes, then use it to customiser yourself those flags in game (as with the in-game dynasty editor you can use any religions flag.)
But CK2 doesn't like flags of the wrong religion, so probably everytime you load a save, you'll have to do it again.
And they can't be of the right religion, as then random baronies and random characters would get them.
 
i usually use the same house colors with as close as possible animals or symbols. i have even made a bastard branch with instead of a white seahorse i used a gray/black one for the velaryons.