For others with this question - I found out that you cannot just add on days and airtime, but you can purchase another 1 year sim, activate when your current one is about to expire, and they’ll transfer your number for free. (Use Twitter)
Thanks for your reply. Unfortunately, it is not possible to add on days and airtime to a preloaded SIM card account. If you own a preloaded SIM card, you may add a payment method to the account in question to ensure no service interruption. If there's a payment method associated with your account, the service will automatically renew at our then-current rates.
Should you rather purchase a new preloaded SIM card, you may activate it on a brand new account and we'll be glad to transfer your existing phone number as long as it is active; transferring a FreedomPop phone number between FreedomPop account is for free. You may request assistance with a phone number swap through any of our support channels, our team will be happy to help!