Joined: Aug 2010
Posts: 12,748
Likes: 2,108
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
I do not think I have ever seen an unthreaded rear fender or rear rack mount on a modern frame, and the ones I have seen were always M5 thread, except some older Raleigh Nottingham frames that I think were number 12 fine thread.
If it is steel, I see no reason that you can't tap it. If Aluminum, it can be tapped, but the metal is softer and maybe there was a reason it was not threaded in the first place.