Can you cover up a window in a bathroom ?

Planning to redo ensuite and have a query with a window on the tiled wall. The window is on second level and a bugger to get to , ideally pull it out. brick it up and cover the hole in wall, bricks hard to get etc. Question is, are you able to take off the trim around the window, sikaflex the window, timber stud the inside opening, then villaboard over the top. Reason is the wall has a small mirror and just not in right place....would there be a problem doing this...there will be an exhaust fan still inside and the ceiling will be created to have a shaft and velux window so it will have the light......