IMEI is hard-coded. For CDMA, it must be added into Sprint systems. Unless GSM, only a SIM is necessary.
I believe someone is giving you false information. You should be able to place an activated SIM card into the phone and start using it. That's how the unlocked Pixel, Pixel XL, Pixel 2, Pixel 2 XL, Galaxy S8, Galaxy S8+, and Note 8 all work. You buy them unlocked and use them on whichever carrier you want, including Sprint. That's how my Nexus 6 worked on Verizon, before I switched to T-Mobile. Verizon also uses CDMA, and they didn't have to add any IMEI into their system. You just put the activated SIM into the phone.
I could take a Verizon SIM and put it into my T-Mobile S8+ right now, and it would activate and get service.
But to answer your original question again, THERE IS NO SUCH THING AS CDMA UNLOCKING. The phone is either carrier unlocked, or its not. And the Verizon Note 8 is 100% carrier unlocked.
I beginning to suspect you bought a Note 8 with a bad IMEI, if you got it used.
EDIT: I MAY BE WRONG (don't tell my wife, she never hears me say that)
After doing some research on Reddit, it looks like others ran into the same issue as you. The unlocked version and the Sprint version will work, but none of the other carrier Note 8s are eligible for BYOD. Which is stupid, since they are all exactly the same hardware.
I'd try this:
Use ODIN to flash either the Unlocked 950U1 firmware, or the Sprint 950U firmware. Then place the Sprint SIM card in the phone to see if it activates upon start up. If it doesn't then you may be hosed. If that's the case, sign up for T-Mobile. LOL.