It can be done, but not the way you think.
On my last trip (solo), my mom decided to join me for the last few days. I was on a Magic Your Way + Dining package. She did not need to be on my package. This is how it went:
When I checked in, I informed the front desk that my mother would be joining me on XXX date for XXX days. I was able to arrange Magical Express service for her as well. When she checked in, they gave her her own room key and then added her tickets.
The only downside is that she could not go on the dining plan (as she was not in her own room for the length of her stay; she was in mine). She paid out of pocket for her meals (3 table service and whatever counter service we ate). BTW, my mom is not a big eater so if your mom is the same, it may okay).
Hope this helps, and it does give you another alternative to consider.