It is a little complicated but I will try and make it as simple as possible to follow. First use WINRAR.exe to open the file data.pak in the folder blitzkrieg 2EXEdata
You do not need to unrar it. The file needs to remain in it's compressed form. We will just modify it and leave it compressed. Make a backup copy just in case a mistake is made.
Now open the Scenariocampaigns folder. In the campaigns folder are folders for all the different campaigns. Choose the one you want to cheat in. I will use the USA folder as an example.
In this folder there are addition folders for all the chapters of the campaign. Choose the chapter you want to cheat and open that folder. I will choose chapter 1.
In this folder is the file we want to modify. It is an .xdb file with the chapter name as the file name. In our example the file name is 1_Bataan_Chapter.xdb
Open this file with Notepad.exe from your System32 directory (you can access Notepad from the Classic Windows start menu and accessories). Now in this file search for any line that says:
The number between these two tags is the number of reserve calls you can make with that battle. In each file there should be 5 or 6 of these tags corresponding with each battle in a chapter. Change them to whatever number you like to have a much greater amount of reserves. I would not change it to anything over 99 as it may lead to corruption of the game.