I've discovered a way to create whole new campaigns for the game:

1.) Navigate to C:Program FilesRandom GamesChaos GateMaps
This is where all the single player campaign maps are. Save them off somewhere else so
you can restore the original campaign later on if you want to.

2.) start up the scenario editor that comes with the game. Under "FILE", click
"Generate Random Map." This brings up the same screen for generating a single random
battle. All the elements are there for designing a particular battle, and the computer
will generate the terrain and the enemy forces. When the computer is done, it will leave
you in the scenario editor so you can make any additional changes if you are handy with
the editor. One quick and easy thing you can do is click MISSION SETUP under the SETUP
button. There you can add a title and a mission briefing. You can also check off which
units can take part in the mission. This allows you to have Captain K and the other high
levels participate in your very first missions. Please note that if you add text to the
mission briefing it will only show up in the mission briefing pop up in the battlefield.
The Cathedral Bridge screen will still show the original campaign's briefings.

3.) When you are done with the generated mission, click FILE and then SAVE AS and give it the
same file name as the map you wish to replace in C:Program FilesRandom GamesChaos GateMaps.
I.e. After you create your first mission, save it as Chapter01.MAP in
C:Program FilesRandom GamesChaos GateMaps. After you finish creating your second mission
save it as Chapter02.MAP. Etc etc etc.

4.) when you start the single player campaign, you will start out at the first mission, get
the Cathedral Bridge briefing for the original first mission, but the battlefield briefing
pop up will show your new first mission. In the addition to the new set of missions you've
created, you'll still get the randomly generated secondary missions too. The thing about
having a new campaign is that Captain's K command will start out with their normal lack of
experience and the new campaigns will allow them to progress just like the old campaign did.
And there's the fun of seeing the marines get picked off one by one as the missions go by.
I missed those two things when I got tired of playing the old campaign.

5.) Cautions. A.) Make sure you have no mistakes (like forgetting to specify an mission
ending condition)in any of your missions because once you start the campaign, the computer
saves the campaign maps off somewhere else as it goes through the campaign. If you've made
a mistake in the Chapter05.MAP, you will have to restart your campaign all over again
after you fix it. B.) In the Scenario Editor's Generate Random Map screen, the size and
power of the Chaos forces are determined by the Level of Difficulty slider. Since the
Ultramarines start at low experience in the beginning, you will have to do some play-testing
to determine what the difficulty level should be for each mission. I.e., if you set the
Difficulty level at 10 for the Chapter01.MAP mission, your guys will get wiped out even if
you have Captain K, the Chaplain, and three Dreadnoughts with you. Match the difficulty
level of the first mission to the experience level of Chapter01.MAP and then step it up
in successive missions.
Have fun, folks! http://gamesplorer.blogspot.com