How can I reduce the size of a bore in steel to accommodate screw threads?
Sounds like you're asking for a drop-in solution for an existing hole and screw size. That's a tall order. I doubt you'll find it, but if you can stand a bit of DIY here are a few options...

A Helicoil (repair coil) normally requires existing threads, but you could epoxy it into place. I'm not sure an M5 will fit your hole, though. You'd want to run your screw in and out before the epoxy sets to clear any squeeze-out.



A threaded sleeve or bushing could work, but again you'll probably need to epoxy it and possibly drill larger.



Tap it for a larger screw. You haven't said what you're mounting, but it may accommodate a thicker screw with or without slight modification. An M8 might work well with the existing hole.