1. Go to the Savegame directory.
2. Edit the .udt file. This contains the statistics of the units present.
3. There are 5 two-digit numbers in the rows cotaining statistics for organization, health, morale, quality and experience. Furthermore there are two digits indicating the number of men in the unit (the first is the initial number of men, the second the present number of men). At the beginning of a battle these are easily to find, as the two numbers are obviously the same. You can then edit each of these numbers. The two-digit numbers shouldn't go over 99, the number of men can be edited at will (I didn't try numbers over 9999 men though). Make sure the second number isn't higher than the first, or an error will occur.
4. Save the edited file and it can be used in battle.