I wanted to replace a horn hook coupler on an all-metal caboose with a KD. The old coupler was on a very small diameter metal post, maybe .050"dia, and the whisker coupler has a large hole. Thought about drill & tap, or a bushing (anything that fits). I put a nut over the post and inside the coupler (#2-56 I think). It fit perfectly.
Do you have any Bowser HO kits around? They supply a sleeve that perfectly fits over a smaller diameter pin such as you describe and the OD is perfect for the opening in a Kadee coupler.
Short of that you could look for the right size Evergreen styrene tubing and make a bushing? I haven’t looked into it but perhaps the diameters will work using an old ball point pen refill (the end without the ink!)
Good Luck, Ed
Kadee #213 bushing kit should work.
Also, I would recommend you use Kadee 20 series (21-29) which are non-conductive. They provide additional short circuit protection for all metal models.
The Kadee #28 has the same working dimensions as a Kadee #5.
I use them on all my brass freight cars and locomotive tenders.
Kadee #28:
-Kevin