I've tinkered with the idea of creating a new SdA.xml, with all of the promos, but people have told me that it would mess things up for those who don't have it. Furthermore, the set names are predefined and cannot be changed.
However, I think that if you want to make your own DC sets it would be best to do it in Lackey. It's really not that hard -- it just involves filling in the stats and game text in a spreadsheet. The nice thing about this approach is that you can have the base LotR Plugin, with all of the real sets, and then distribute the dream card sets separately. Furthermore, if you wanted to play with a friend who didn't have the data files for your dream cards, they would be automatically downloaded when you connect to them.
I'd love to put up some Lackey tutorials, but I'm waiting for a) an updated LotR plugin with all of the new cards and fixes and b) a certain program bug to be fixed by Trevor that can affect gameplay with proxy servers (i.e. when not forwarding ports). Well, and also c) time.