I believe its been done, might have to go to a Sprint store.
Just to elaborate on that,
I had a sprint business plan & was trying to join a framily plan, however when i called up sprint to join i was told that i have to downgrade to a personal plan & then switch to the framily plan.
After hearing that i went over to the sprint store in Boro Park where the rep told me that's nonsense & instantly joined me to an existing framily account. Now i must stress that i was out of contract, the rep told me that those in contract who are looking to switch can do so but have to open a new framily plan.