Can I replace ceramic bearings with steel bearings?
You haven't checked inside the hub but are planning on a possible downgrade?  Best to open it and check first.

I don't personally own anything ceramic, but one of their main advantages is durability.  I've seen estimaguesses of 50k-100k km life for "quality" ceramics.  Its unlikely Campagnolo used knockoff bearings from new.

Answer: Yes, but check inside hub first.  No point fixing a non-problem.