This is what Darksaber has to say about it:
Darksaber wrote:We've been working hard to make the X-Wing Alliance Upgrade Craft Pack (XWAUCP) compatible with STEAM's version of X-Wing: Alliance. I can now happily say that you'll be able to install the XWAUCPv1.3 with your STEAM XWA.
You may have read that the STEAM version will not be supported, Well, sorry about that, I just needed some time to figure out a workaround to make the XWAUCP compatible.
I've also added General_Trageton's NEW Assault Transport
So what are you waiting for? Download the NEW XWAUCPv1.3 now and experience XWA with all the Upgraded Craft.