When did that get added?

Was it with RotT the entire time? I vaguely remember asking about how to set up Extreme with the GOG installer.

you can select the extreme levels - use modified stuff - choose alternate game levels.

http://www.3drealms.com/downloads.html

I don't know either way if the freeware release can work
with the GOG release....

GOG already features every level pack in that list except #25 - User levels for ROTT that have Blake Stone geography. You would just need to extract the files to your game directory and run the batch file for that one.