How do I define custom properties in Roblox-defined tables?

Instance.brandNew = Instance.new

So I can do

local part = Instance.brandNew("Part", workspace)

Attempt to write to a read only table error. How do I overrite that? Similarly,

Faces.insult = function(insult)
	print("Your FACE is a '..insult)
end

How do I do something like that?

I'm pretty sure that you can't. You can however use a ValueBase intance to parent to the Instance instead. (StringValue, BoolValue, NumberValue, IntValue etc.)