What I want to know is which of those exogeneous peoples brought in the skirts?
All of them.
Skirts are easy to make and easy to wear. You can relieve yourself without disrobing and one size fits most.
However once horse back riding arrived things changed and breeches were born. More guys rode then girls so breeches became a "mans clothes" and skirts became "girls clothes".
Where the weather was hot or there was not a lot of long distance horse back riding skirts stuck around long enough to move into the category of "native dress" and survived.