Can I replace steel bearings with ceramic bearings? I'm using WH-550 from Shimano, can I replace my steel bearing balls with ceramic bearing balls?
Yes, you can. You just need to get the right size of loose ceramic ball bearings as that's what shimano wheels use. They are available from various vendors, like here for instance. 

You didn't ask this, but I'll answer it for you anyway: would I recommend replacing the bearings on a pair of WH-R550's with ceramic bearings? No, and neither would many others. Ceramic bearings are expensive and can end up costing nearly as much as your particular wheelset. There are better places on your bike to spend that money and get more bang for your buck. A lighter, more aerodynamic wheelset would be a great place to start.