So I wanted to add my fourth add-a-phone (maxing them out) after buying the last Sanyo 4900 in town (closeout at OfficeMax – $119). I went into my Sprint Store and suprise: you have to sign a one-year agreement.
Determined, I called *2 from another phone, same result. Then I called the activations phone number, and they asked “what contract do you want, one year or two?” I responded “I already have a one year advantage agreement.” the CSR responded “oh” and proceeded to add the line, sans contract.
Bottom line: you may have to try more than once, but it is still possible to add-a-phone without having to sign a new (dis)Advantage Agreement on the line.