You can do it in one of two ways. (1) You can edit the save game files and insert leaders for yourself. This edit only lasts and works for the save game you edit. (2) You can edit the leader files themselves, which will permanently include the new leaders in all games.
To do (1), you have to open the save game file. Go into the main EU directory and then into the /db/leader directory. At this point, I would suggest making a new folder called "Original Leaders" and copy and paste all of the leader files into this new folder. Then go back to the main leader folder. If you wanted to add leaders for England, you would open up the corresponding leaders.eng file. You have to use a pure text editor like notepad or wordpad to do this. Once the file is open, you can edit any of the attrivutes of any current leader. To make a new one, you will insert text that looks somewhat like this:
Code:
[color=white]
historicalleader = {
id = { type = 6 id = 5000 }
category = general
name = "Cow Pie"
startdate = {
year=1492
}
deathdate = {
year=1845
}
rank = 0
movement = 24
fire = 12
shock = 12
siege = 9
}
[/color]
The id number must be different for every new leader you add. I would sugget using the 5000 range and up. The highest number you can use for stats is 24, anything higher and the game wil crash. If you want an admiral or a conquistador instead of a general, just replace the "category = general" with cetegory = admiral" or whatever kind of leader you want.
When you're finished, save your file, and you're good to go.
The same process I described above can be used for editing a leader in a save game. Find your savegame in the EU/scenarios/savegame directory. Open the corresponding save game file in wordpad or notepad or some other text editor and find the section of the save game that corresponds to the country you are playing. You can then do the above procedure and you'll be fine.
Hope this helps. Good luck editing!