If we are talking about sounds, this is how you do it...
1) In the campaignfolder you make a separete subfolder named DtaExt. Then you place a file named vssver.scc in the root folder of the campaign. You can copy the one in the CWC1985 campaign.
2) In the DtaExt you place a folder named Sound. I will now refer to it as DtaExt\Sound. Inside this new DtaExt\Sound you place another copy of vssver.scc. This file seems to be the one that enables the different subfolders to "look into" each other during game play. I don't know why, but it will not work without them.
3) Then place all sounds you want to have access to in the new DtaExt\Sound folder. This folder is universal and all missions defined in the description.ext of the campaign can now reach them (even if the are not defined in the description.ext belonging to the individual mission itself).
4) Define the sounds as sound, radio or what-ever in the description.ext that goes with the campaign. And make sure they are only defined here. If they are defined in the campaign extension and the mission extension simultaneously it will not work.
5) You call the sounds from within the mission by refering to them normally. Example: bedges say "yesItWorks"
I've only done this with sounds and music that takes up a lot of space, but I guess that the same principles works for images, dialogue classes, et cetera.