Ah yes, it is not a patch. It is the IGC. Improved Grand Campaign
It makes the game even more historical by adding new nations, leaders, re-dividing lands etc.
Note that it will change all EU data so all scenarios will be altered a little. But don't let that scare you, almost everybody over here plays IGC only.
Just unzip it in your main EU directory, run patch.exe, then run IGCConfig.exe. There you can choose all options you desire.
Pick countries etc. If you want all countries to be annexable, simply make only one country playable (yours

)
Here is the site. IGC 2.3 is latest version.
I also advice to download the latest EU patch, 1.09
It can be found on top of this page, under downloads
http://www.ludd.luth.se/~doomdark/EU/IGC.html