I have replaced the rear non-drive bearing of a Dura-Ace hub with a cartridge. I think the replacement was a 6001-RS; anyway it fit perfectly. And it functioned perfectly. Of course, I needed a threaded sleeve to support the bearing (12mm I.D.) on the 10mm threaded axle. I also needed some spacers and a dust cap that was spaced about 2mm outside of the cartridge to provide an additional layer of protection. Cartridge bearings are in fact very poorly 'sealed' for bicycle use, so you need to create a second labrynth seal to keep the worst of the water and dirt out.

Keep in mind that the cup dimensions of XTR and Dura-Ace hubs are different than anything else in the Shimano lineup. You have to measure...

And don't put too much side preload on the cartridge - the standard 6001 bearing can take a certain amount of preload, but don't overdo it.

On decent Shimano hubs I have also replaced the ball cups. I had another Shimano hub with the same dimensions; I scavenged this for the cups, balls and cones. You need to rig out a cup extractor, and some way of immobilizing the hub shell while you hammer away during the extraction. And then a bearing press to push the new cup in. This is the better solution, as Shimano's hub design is superior in every way to any hub that uses cartridges.