A plan is part of a phone line. You can have one account with several lines. So you can add a new line to your account and choose the plan for that line.

I'm assuming you mean you and your wife share 1 account with two lines? You don't share a line unless you both are sharing one phone.

@Grizz23 You currently have one account with 2 lines.  Both of you happen to have the same plan. Adding an another line will require picking any currently available plans. Based on what your daughter’s needs maybe. There are some very good promos available right now, especially online.