Resealing bearings in chinese OSPW

While I have no practical experience trying to do this, I doubt that what you propose is easy, and it may not be possible.

Rubber seals require a lip machined into the inner and outer races. This retains the seal. I suppose you could check for a lip. If the bearing were designed specifically as an open (i.e. un-sealed) bearing, which is sensible for a low-contamination environment, then it likely wouldn't have a lip. If it did, you would then have to source a seal with the correct dimensions to fit the lip. There are probably a number of different geometries of seal lips. I don't know the descriptive terminology or important measurements for seal lips and grooves. If you found these out, I expect you'd then have the problem of buying just two pairs of seals - the entities selling those seals probably aren't set up for retail transactions, since really what consumer does this? So, it might be several hundred seals.*

It should be difficult, and it may be impossible, to extract the balls from the cartridge. The balls fit tightly in the races, and in many cases, I suspect they sit in grooves in each race. For example, see the second picture at this link. If you can’t pry the bearings out, you would need to disassemble the cartridge to get at the balls. Furthermore, cartridges aren't designed to be disassembled, so I think this process would be destructive, i.e. you'd need something like a hacksaw. Getting the balls into a new cartridge could require industrial machinery like a hydraulic press. And finally, remember that a bearing series number like 689 just describes the outer diameter, the inner diameter, and the depth. The race thickness can vary - so, thinner race means larger balls and vice versa. You'd need to find two pairs (again, consumer vs industrial scale problem) with all the important dimensions matching your ceramic balls.

Finally, ceramic is relatively hard. I believe that best practice might need you to specify an appropriate race hardness as well (it may be more important in wheel bearings, since as you go over bumps the balls might indent softer steel races). I am not sure how this quantity is parameterized in the industry (i.e. maybe it's Rockwell hardness or something like that, and you need to specify a minimum hardness, and I don't know the appropriate minimum). On another tangent, be aware that the quality of ceramic balls does vary, and I suspect cheap ones are low quality. Plus the quality of the steel used in races also varies, with cheaper races probably having poorer material qualities (probably stemming from more impurities, poorer manufacturing processes) and probably looser tolerances.

This answer is written with no practical knowledge of manufacturing or remanufacturing cartridge bearings. Basically, I learned from a podcast. I could be wrong! And if I am, you should free to let us know. However, hopefully this post has given you some idea of the possible pitfalls you would face.

*I am aware of one company that sells replacement seals for its own bearings: Kogel, which coincidentally makes jockey wheels and bottom brackets with ceramic bearings. Kogel's instructions suggest annual regreasing, which is why they sell replacement seals (although they have communicated to me that if you carefully remove the seal, it's usually reusable). I haven't asked, but I would suspect Chris King sells replacement seals for its bearings, although they have seals that are designed to be repeatedly removed for regreasing. The issue is not that no seals for cartridge bearings can be bought in consumer quantities, it's that you will probably only find seals matched to specific bearings that are designed to be serviced, and this seems like the minority of sealed cartridge bearings.

I assume you mean the bearings from the pulley wheels. You could pry off the seals using a knife or something similar.

I would add low shear grease (such as freehub grease) or oil if you want even less friction.