When you have a prepaid account, you don't have an actual "account" where you get a monthly bill or anything like that so you couldn't "add" a phone, per say. You would just have another individual plan whether it has an actual bill or is prepaid.
I work for Sprint now but I used to work for T-Mobile. I always found that people who are on prepaid accounts end up spending more than people on a monthly billed plan. You may want to think about putting both your Dad and yourself on a monthly plan as I guarantee it would be cheaper.
Send me an email if you have any questions.