So, say I have a Consulate Dreadnought and I enchant it with Siege Modification. The Crew effect makes it to become an artifact creature, but if it is already an artifact creature, does it need to be crewed to attack?
No it does not. Siege Modification says that it's a creature in addition to its other types, so it counts as a creature for anything that cares about it being a creature, including the ability to attack with it.
This is the main point of Siege Modification's ability.