How to create a custom unit based off an existing unit in the game?

Hi, i’d like to know if it’s possible to create a custom unit to use for custom missions that is based on the data of existing units in the game extrated from the unpacker. forexample, changing the engine values of a plane of tank, changing their armament and so on.

Or is it impossible and you need to create the unit from scratch? because I’ve created custom units from their blk file, but as soon as i change a value in them, i can’t load the mission