Oh snap, we hadn't thought of using them that way before but it actually sounds pretty fun!
If we wanted to run HUNTERS as a supplement for players instead of opponents, we'd probably:
* Using it in Campaign play, each time a Runner completed (and survived) a run, we'd award them 1 rank on their chosen Hunter Archetype (so, after completing the first mission, they'd be able to choose between either of the Rank 1 upgrades). If playing a REALLY LONG campaign, we'd make it so each successful mission added 1 mark to that Rank's progress track instead, making Runners unlock the upgrade once each track got completed.
* If using it for one-shots, we'd do... Basically the same thing, but instead of making progress between missions, we'd have Runners attempt to unlock them during prep time for the run. Runners could make rolls before the run started to attempt to unlock the Hunter upgrades - but that would also mean risking starting the run with the Stress bar pretty high, which would make the run even more dangerous! (we would also consider adding tougher obstacles according to how many Hunter upgrades the party unlocked for each run - they wouldn't bring all that gear if the job wasn't as demanding!)
Glad you liked the addon and thanks for the question! If you end up using HUNTER UPGRADES for your runners, let us know how it went!