Freedompop's customer service isn't great and I haven't tried this with freedompop specficially, but I did it with Ting. I called Ting's customer support and they moved the imei from the prepaid database to the post paid one and I activated it on my postpaid count immediately after. If freedompop is giving you trouble you could try activating it on Ting, then immediately call them and have them return the eligibility for a postpaid account and hopefully you can activate it.