Actually Morten,
Yes. You can give a civ extra units to start with but it is tricky. Here's how you do it.
First, you will need to start up the scenario in the civ you want to begin from in the scenario (usually, I have a civ that I place all units and final changes in last so that no moves occur while I am adjusting other civs).
Second, use the cheat menu to access the 'reveal map' and select the view of the next tribe. This is very important that you go down through the reveal map menu one tribe at a time until you get to the tribe you want to view because otherwise the computer will automatically activate a turn for every tribe you skip past!
When you get to the tribe you want to add units to then do so.
However, you will still need to go back to the 'reveal map' menu and cycle back to the tribe you wanted to start with or turns will continue for the rest of the tribes when you start the scenario back up again!
Holy moly...it's a bit time consuming but this is the only way to do it clean. I learned this trick when I designed "Birth of America" and found it to be invaluable in making last minute changes to a civ that was imbedded with a lot of cities, units and whatnot.
Experiment first with a practice scenario and then try it out. Tell what you think.
John