As Bitcoin core v0.16.0 released You can import Bech32 Segwit address as @Willtech Answer.
importprivkey "privkey" ( "label" ) ( rescan )
First Make sure that you are running the correct bitcoin core version
Now set address type to bech32 in your bitcoin.conf file
so when "importing" a new private key, it creates a totally new wallet
file? why cant it add it to an existing wallet? Also does this
"import" know to import the 3 derivatives of addresses? 1xxx, 3xxx,
and bc1xxx from a single private key?
- No it doesn't create an new wallet file, it will just add it to your existing wallet file.
This way it should work 100% anyway if it didn't work. you still create your transaction manually and sign it using your private key.