First you have to extract the base game maps using a tool, GrimArc can extract the files and use MapDecompiler inside the extracted folder .
You need to extract the Levels.arc located inside the gdx2\resources directory , after you extracted and used the MD in the extracted folder .Create a new mod +go to source and copy the levels into it , also create a maps folder if it does not already exist and copy world001.sd and world001.wrl into it.
That should be it but you do not have to do all that if you download this [MOD/RESOURCE] Campaign Map (Template)
Good Luck .