We already solved that part - here is an updated NCF patch that you can use with Deflaktor's app.
Inside you can change the amount of total units from 1000 (where the last slot is always reserved for "rotate all" or something like that).
The patch also allows NCF creatures to be used to create a map with RMG without crashing the game.
With the knowledge we have we believe it is possible to add new item slots in other domains - skills, spells and may be towns so we want to find out how to be able to add them in the editor too. That is why I ask about changes in index.bin
Here are a couple of images from RMG: