the mod contains 'story code' and therefore requires starting a new game unfortunately. Story code is only read once from the game files and then stored in the save file (and save file editing may be risky, I'm not familiar with that; it would require changing the database entry for the party limit, changing a dialog character flag if the limit has been reached in the current game, and maybe more).

My mods for DOS 1 EE: FasterAnimations - QuietDay - Samaritan