Updated to 1.1 to look for sod-dlc.zip where GOG puts it.
People have certainly made their voices heard about the zip based dlc that we use on Steam and Gog. I had a few hours to spare Saturday and Sunday night, so I made modmerge for those who may be interested.
This comes with no warranty, and if you need to restore your steam copy to normal, you can use the 'Properties->Local Files->Verify Integrity of Game Cache' feature to restore things to how they were before.
If you download this and place it beside your game data folder(aka, modmerge.exe should be sitting beside chitin.key), and run it, it will do the following.
Backs up your chitin.key to chitin.key.bak, just in case
Unzips your dlc/sod-dlc.zip, copying over top of the existing lang/, movies/, music/ folders. The data folder inside of the zip will be renamed to no overwrite files in the existing data directory.
Updates your chitin.key with so any resources from inside of the zip file will point to their appropriate bif files.
Renames your dlc/sod-dlc.zip to dlc/sod-dlc.disabled so the engine no longer loads the zip based dlc.
This should give you a chitin.key that references all the BGEE assets in data/*.bif, along with referencing all the SOD assets in the sod-dlc/*.bif files.
I've tested this on two machines here(PC/Mac), on steam. Let me know if you have any success.
@elminster 's edit: Mac users should make sure the "modmerge-osx" file they download is renamed as "modmerge" before trying the above steps.