Can’t “Save To Mod” Plugin Content

I am using MechWarrior 5 Editor, Unreal Engine 4 Editor prepackaged with a special project file, to make a mod that builds off another mod. I have the source files of the other mod in the plugins folder. The associated plugin is activated. I can see and edit the files from the other mod.

I can’t “Save to mod” anything from the other mod’s source folder. Nothing else is affected. Why?