Are you getting the dining plan? Will she be using ME?
Things are easier if you aren't using the dining plan or if you don't mind her not being on the dining plan. In that case, just talk to the front desk when you get there and they can take care of adding her to the existing reservation for only the nights she will be there. You can't do this ahead of time because the way the packages are set up everyone in the package has to be on the reservation for the entire time. That means if you are getting a dining plan, she would be included on it for the entire time. If you add her to the reservation when you arrive, they can add her easily to the room reservation for the nights she is actually there, but she won't be able to have the dining plan. You can either pay for her meals out of pocket or have her on the reservation for the entire time and use her dining plan credits yourself.
ME is another issue. Once she is on the room reservation, you can call the DME and have it set up for her. If you do this at checkin, it would be too late for her to get luggage tags, but she can provide the DME desk with her luggage claim tickets and a description of her bags and they will go get them for her.
If you have the dining plan and want to end one package and start a different one that includes her, you would have to buy a second set of 1 day tickets to get the dining plan (or 2 day tickets if you have the
free dining offer).