There are some (older) handsets that can be used directly with Boost, but (AFAIK) the TouchPro2 was the last and even those were hit or miss.
To get anything newer (such as your wife's Epic) you need to use the serial number from a "donor" phone that is a genuine Boost phone (such as the Incognito).
You will NOT get 4G with a donor serial number.
You will need to cancel Sprint in order for the serial numbers to not be "in use", or you could try to call Boost and port the phones and numbers directly. I did that while I was still on iDen with Nextel, took less than 5 minutes to make the switch.
If all you want is voice I would seriously look at just picking up a real Boost handset locally.