If you did it that way, you'd have to go through and do that for every member of that faction that you put in to the mission/scenario. Then if you used all the information exported from the Arsenal's export function to get all the class names, without using a script or [] call fnc_XXXXXXX, it would clog up every unit's Init field and slow down the mission processes and loading.
I would recommend trying messing around with creating a script for changing the entire faction's BLUFOR/OPFOR/INDFOR allegiance or a specific groups allegiance, and then utilizing a [] call fnc_XXXXXX to stream line the process. Yes, it may take a bit longer to get all setup properly, but once it is, you can use it again and again with ease. I'm actually in the process of trying to figure this out myself, because I want to use the FIA as an entire faction of separatist guerrillas without having to grab a bunch of crazy mods and what not.