I think that you can just purchase the pin directly from BPA. I don't know if there is a ceremony done for this. Perhaps one of our more seasoned alumnae can tell us?
The Mother's Pin has been around at least since the 1930s, since it is shown in my 1931 History. I have been meaning to get one for my mom for the longest time, but I'm not sure if she would get much opportunity to wear it!