Unverified plus you want to have a copy of your original game file that's going to be hex-raped for the hack.

Go to your DOW directory and select a profile (you can do this with a new profile if you want). Change directory to the .../wxp/singleplayer/campaign/... and select either folder state1 or folder state2. State1 and State2 correspond to Order and Disorder campaigns.

campaignstate.lua (in the state1/2 folders) is the file you are going to alter (back it up stupid). Use Notepad.exe to open it. Locate the strings "maxmission=#" and "scenario=#". Change the "#" to a whole number corresponding to the latest campaign stage you want. Keep in mind that 12 is the maximum value you can safely use. Naturally, you need to save the file as a .lua file, not anything else.