Yes, you can do that.
The first line in the file is just a count of nations. So you would need to change this to 3 if you're adding just one.
Following that you need one row for each nation, to define the side, nation short name, and soldier ranks. Despite what is in the workbook the fields of these lines beyond RANK COY CMDR are not actually used.
Side = 0 for Allies, 1 for Axis.
The nation name is a two character string. This is used to locate the soldier names data files (i.e. USNames.txt, GENames.txt) and the soldier voice cues (Like GEVox.sfx and EGEVox.sfx for the Germans - DefaultVox.sfx is the US voice cues and is used by default if there are no separate cues available).
The ranks give the base rank for a soldier in each position.
The second section of data also needs one row per nation, and gives the unit numbering string for units at Platoon level and below. These need to be in the same order as the nation data in the first section. The first field is the index position of the nation, and should just go up from 0 for each row. The second field should always be 0. The remaining fields are the 20 position strings used to name units by default (i.e. 1st Squad, 4.Zug, etc).