There is a way to do this, sort of, but it takes a bit of fiddling.
You start as usual, buy force, and deploy. Then, while still in the deploy phase save the game. Rename the save files to change them to scenarios. There should be 2 files, a .dat & .cmt. They are named SpSvXXX.* Change to SpScnXXX.* .
Open the mission editor and load the "scenario", pick/purchase enemy force and auto deploy. (Can send and have someone else deploy too, for best results.
) When done rename the files back to save format, load and play as normal.
I haven't run into any issues when I've done this, but, do at your own risk.
Re: Preset Campaign Picklists
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