From what I know from using the editor, modifiers cannot be range dependent. A stat change affects all ranges equally.
Exactly, that's what mean. The basic weapon itself has its "true" (which is to say, in actual words instead of game mechanic words) rate-of-fire vary greatly based off range which is currently definitely feasible, and then a suppression modifier would cause average suppression per second to vary like the basic weapon's rate-of-fire based on range.
So hypothetically, make weapon have a "true" rate-of-fire of 0.5/1/3 per second. Thus the weapon can get a value of suppression from an additive modifier from the ability (say, 0.05 per shot), and then do 0.025/0.05/0.15 average suppression per second.