Hard way: Move the appearance.2da file out of your override folder and then run the Arena mod installer. When it's done open the appearance.2da file it has placed in the override folder with KotorTool and manually apply any changes (changed cell values or new rows) that the other mod had made with the old format 2DA file. Without knowing which mod it is you have I can't say exactly what you'd need to change.
The arena mod will not work properly without the updated version of this file. The game will crash when certain combatants spawn if it's not present.
Ahhhhh... *sigh of satisfied curiosity*
I sent you a PM asking this very question, Stoffe, before I found this topic. (includes link to the mod (
http://knightsoftheoldrepublic.filefront.com/file/Slender_Body_for_Females;41158) I want to add.) This clarifies whether or not the file is important. But now I need to know how to do things the hard way.
Any help would be appreciated.