I ran into this too the end of last year when I wanted the Hero phones. Moved me to the 1600 minute referral plan--similar to the Sero but allows the newer phones. A couple months later, I tried to get a third line added via a deal from an authorized Sprint rep but they couldn't put it through. Radio Shack couldn't do it either. Turns out that I had to come off the special 1600 referral plan and change to the 1500 minute plan because they couldn't properly access the information in their systems. I guess their system is slightly different than Sprints. The deal for the phone was about $100 less than what Sprint wanted so, I changed the plan down and got the additional line added through the Sprint third party/agent (I think it was Radio Shack).
Overall, the skinny is that I moved from 2 individual Sero plans ($50ea--overall about $115/mo or so--can't remember the number exactly) and changed to the regular 1500 min shared plan; added a third line and with my discount comes to about $140/mth all in! That's 3 lines and all the goodies for just $25 more each month. With the cell phone to cell phone not charged against the minutes, we barely use 700 charged minutes, whereas we used to use about 2200 charged minutes on the other plan.
I decided to stay on the 1500 minute plan because they can actually take off the discount (23%) whereas the 1600 minute plan it was already discounted. Apples to apples (almost) it was actually cheaper to stick with the 1500 minute plan with my discount (by about $10/month).