Hey there, I find that when I'm missing a file, i find it from the ARMA 2 Folder. For example, you go into steam and find the original ARMA 2 addons folder and find the file you're missing. Then put it into the addons folder of the ARMA 2 expansion folder. I'm not at my computer so i can't give you the direct file path. If you find a major issue with a whole bunch of files you can add all of the addon files from your original game into the addons folder of the expansion.