X visa extending.

Mali, I don’t think you can extend it for a year because you have finished your study. If you need to do internship for a year, you may need to apply for a work Z type. You can go to the exit and entry administration of the local PSB and consult if you can convert the current student X type to a work Z type, if yes, just prepare the required documents and change it. If not, I’m afraid you need to go back and apply for it in your home country.
The required documents include the work permit, invitation letter of duly authorized unit (your employer will get the two documents for you), your passport and its copy as well as the application form affixed with a passport-style photo.