Is it a common practice to copy code from the .net libraries?

Is it a common practice to copy code from the .net libraries?

I once saw a project in which at the root element the programmer had
folders like MS.Internal or Microsoft.*. He had copied the decompiled
source from the .net classes in classes not marked as sealed to modify
them or to be able to access them or modifying them in another way. I
wondered wheter this is the common way if there is no workaround for such
situations or whether this is even legal?