FreedomPop runs off the backend of Sprint meaning any Sprint or FreedomPop device/services are compatible with one another. FreedomPop runs off the CDMA technology.
Since you are wishing to switch carriers and bring your current device (Nexus 5), you should first make sure your device is unlocked! Also, make sure you have no outstanding fees on your device or you won't be able to unlock it. With the FreedomPop device being unlocked, you have a lot of available service options.
Here is a search results page of all the compatible carriers with your FreedomPop phone
Each carrier is compatible with your device making the switch super easy! If you are wishing to port your number, make sure you do not disconnect your current service linked with that number or it will be near impossible to get back.
From these results, you will be able to compare data inclusions, minutes/messages and prices to choose your best plan.