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.
joelmdev's answer is correct. Just to add some very minor additional information: some high- and mid-quality wheels have cup and cone bearings. To my knowledge, this applies to all of Shimano's factory wheelsets and higher-end wheels made by Campagnolo/Fulcrum (NB: Campagnolo is Fulcrum's corporate parent). Most wheels have cartridge bearings.
If your wheels have cartridge bearings, you can replace all the cartridges with ceramic bearings. The only thing stopping you is the cost-benefit ratio. As correctly noted, that ratio is small. Note that this would mean replacing a total of up to six cartridges, two in front, 3 in the rear hub, plus one in the freehub. In my judgment, the WH-550 was a low- to mid-range road wheelset from Shimano. Changing to good ceramic bearings may save 1-2W of friction, and they may be more durable. It would have been better to upgrade the entire wheelset. The drag savings from an aerodynamic wheelset are likely to be larger.
If you have cup and cone bearings, then the ceramic balls are harder than the bearing cups and cones, possibly much harder. I believe that the races in hybrid ceramic bearings (i.e. ceramic balls with steel races) may need some minimum hardness, or else the balls might damage the steel races over time. So, while you could physically change out the balls in your wheelset, you obviously aren't changing the cups and cones. This could wind up being counterproductive in the long run, as opposed to just a very poor cost/benefit action.