Using the editor it's very easy to alter existing cup competitions but there's doesn't seem to be a way to add completely new ones, unless I've missed something obvious.
I would imagine directly editing the .xml file would work but it's a little over my head and I don't really know where to...
It's a rule that's coming in to effect in the Premier League from the 2010/2011 season to ensure there are more 'home-grown' players in the Premier League.