Hi,

Like Mickael said, a boolean with sphere could be use.
I had used a boolean merge to begin the corner and delete the 3/4 sphere useless.







Next I deleted the smooth part of my quarter of sphere.




I selected the object to do an offset/shell and use an flip paramater to have bigger size than the original corner.








After I selected edges to filet and have a smoothed corner.















The object is a Solid so is printable but I have two small artefacts due to filetting.