Question Regarding YAML and How to Use the Import/Export Feature

Hi! What it says on the tin. I'm trying to set up a custom campaign start for some friends and I - was curious if the Import/Export feature in YAML would allow personalized mechs without breaking the game.
I have no idea how this feature works and from what I read its based on MWO's system but I never played that either. I'm thinking perhaps a Save Editor is perhaps my only option. Any help would be appreciated!

For future reference, I have YAML and Advanced Career starts - at least. Without divulging every single mod I have. For time purposes.