Why would you need to edit the module's model? You would just add a line item in the import CSV with a minimum weight of 50000 (assuming you are using grams) and include the surcharge in with the shipping price for that weight increment and any above it.
I've included a screenshot of one of my Matrix Rates tables, if I needed to do what you are suggesting I'd just add a line with a Weight >=
of 50000.