Easiest way to pick out a character isn't by name (which varies, sometimes wildly, based on localization and character culture) but rather by their numeric ID in the code. From the console in the game, enter "charinfo", then mouse over the character's portrait until the debug info pops up. Charlemagne has character ID 6392, and you'll find him (like most early Frankish characters) in the characters file "old_frankish.txt" under the base given name "Karl". Once you've done this, you can copy the file over to your mod in the appropriate directory and add the code ZeroGravitas already posted.
I'll admit, though, I did cheat a bit to find it efficiently: I grabbed the ID and then did a search for it through all the character files using Notepad++. It seemed a touch more efficient than poking around manually.
EDIT:
If all you have is the display name, though, there is an alternative method of pulling his character ID without opening the game. All you need to do is identify the file containing his plaintext display name in the localization directory. For Charlemagne, he is located in the file v2_50.csv. ERA_CHAR_NAME_6392 is set to Charlemagne in English and French, Karl der Große in German, and Carlomagno in Spanish, where the number on the end (6392) is his character ID. This is obviously a bit non-obvious, if nothing else because Charlemagne DLC was released concurrently with v2.2 and not v2.5 and because Charlemagne was in the game on version 1.0 release. There are hundreds of localization files and not much to easily identify which one you need, so some way to search inside the files is more or less mandatory if you go this route.