There should be a button at the bottom of the sov selection screen that says create sovereign. This will take you to the sov creation screen. After you have created and saved your new sovereign. You should see the new sov as a selectable sov on the selection screen. Simply select the new sov and press next.

I see. There is not a way to do this in-game, but it is a fairly easy thing to do with a light weight mod. Let me show you how to do it.

You will need to create two files for this endeavor. The first we will call Mod_NewRaceConfig.xml and the second we will call Mod_HideCoreRace.xml, though the actual names are completely up to you,

In first file, we are going to create a mirror version of the CoreRaceConfigs.xml file found in "install directory/data/english". Now, we do not need to copy all of the factions over, only those which you wish to modify. For our example we shall assume that you only wish to modify the kingdom of Altar. So we will copy the entire file contents and remove all other factions definitions save for the one for the kingdom of altar. Now that we only have our desired faction, we will first change the internal name so it does not conflict with the normal version. After changing the internal name, we will edit two specific tags.

We will modify the leader name to be some sovereign's name we created for this example we will call him "Examplar" and thus we should have the following.

Now, it is assumed that Examplar was created in game via the sovereign creater, and so we will find the file for this sov in the "my games/elemental/Units" directory. In this file, we will take the internal name for the unit, which is found in the first line of the unit definition. Going back to our mod file, we will change the line:

to use our new sovereign's internal name, which will be something like Examplar_xxxxxx-xxx.

One we have made these changes, we can save the file and place it in the my games/elemental/mods/data directory. Our second file will contain the following without modifications.

This will hide the normal version of Altar in the faction selection menu. Leaving this out however will allow you to select two different versions of Altar, the normal one and the modified one. Once you have added this text to the second file, save it next to the first. Now you should be able to run Elemental and have your custom sovereigns instead of the default ones.

No problem. I'm glad you can play the game in the way you want to play it.