Importing master keys on bitcoin core
You cannot import private keys to the keypool. This is currently not allowed because it is generally regarded as unsafe, at least with the way that the wallet works today. There is ongoing work to change this in the future.

You could do a split wallet setup where you have a watch only wallet with the pubkeys imported into the keypool and use that for address generation and transaction creation. Then you have a wallet with just the private keys imported and you use that for signing. But that's not what you want.

If you aren't tied to using a BIP 39 seed, you could generate a 256 bit seed and use sethdseed to set the HD seed of a wallet. That would do what you want, but BIP 39 uses 512 bit seeds and not 256, so it won't work there.



Imported addresses are never automatically used when you do getnewaddress. When you import them, you need to set keypool: "true" for each imported object for that to work.