CampaignExtra Mod (Zero Hour) 

So to introduce custom units into a .big mod, just use finalbig to add in the custom files?

To mod files inside the .big, extract the file out, modify, put back using finalbig.


Is that correct method?

Yes, but if you only want to make small changes it's probably quicker and easier to modify them in finalbig. Also it's generally suggested you finish editing everything before packaging the mod in a big.

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Its a mod from this site actually, and I've just put back all the modded files.