The feature already exists. It is called a Glyph transform script.
These scripts will change the weight of a font by adding pixels to either side of strokes. They don’t currently differentiate between thick and thin strokes, just crudely adding pixels either side of existing strokes, and apart from the Italic script, they do not modify the Naming fields to change the font style.
So there is room for improvement here, but the scripts are editable so you can modify them to get better results with the particular fonts to which you wish to add a new weight.
Please see my tutorial on Working With Transformations