No, i imagine it would be a side-campaign to your main campaign. It would do as Nokhor suggested, with a notice notifying you of many harrowing months at sea and the discovery of a new world, then take 10% of your forces and give you the option of switching to and fro from the main Euro campaign to the America's, where many other European nations have established towns there.

Or, to make the Aztecs interesting in SP, they'll have many other tribes to fight, like Red Indians and Mayas.