Quote Originally Posted by POL News page
FFXI: Ultimate Collection Seekers Edition Coming Soon to Steam (03/26/2013)
As we approach the release of FFXI: Ultimate Collection Seekers Edition, we wanted to provide an update to our customers in regards to product availability on Steam. We are working at full speed to ensure that we get FFXI: UCSE available on the Steam store as soon as possible for those of you that prefer to purchase games from Steam. However, knowing that there is a possibility that the Steam version will not be available for purchase on launch, we wanted to alert you as soon as we could.

We apologize for this inconvenience and will be sure to provide you with an update on the progress of the release of FFXI: UCSE on Steam very soon!
Expansion should be purchasable for steam users but may not be immediately available at the time the expansion goes live, according to the news post above.