NCF – Extended list of Neutral creatures.

What program did people use to open Game.exe and add new creature names?

Creature names are defined in .txt files, and the references to them are in .xdb files. You can make changes in game.exe [using any hexadecimal editor, i.e., without decompilation]. The maximum possible number of creature IDs, which are described in the corresponding .xdb files, has simply been increased.