hi arsene,
yes its possible to replace the crystal. my friend dropped his hamilton watch and entire front crystal cracked. send back to swatch group service center. the problem with theae crystala are that they are quite hard to find here. in my friend case they have to cut rhe crystal to size again from overseas takes about 3 months. but costis quite fair about rm300.
this is for hamilton.
for seiko u can try to go back to the shop u bought it from and ask them to send for reapair to replace the crystal.
alternatively, if its just a scratch u dont have to replace the whole thing, send to a independent watch repair shop and if they have the right tools thy can glass polish to remove the scratch
hope this helps