Can you turn a private key into Bip39?
private key example (just generated it, so it's empty): xprv9s21ZrQH143K31B6HriXRx2YFmBjyvUA55YjK5c8bKBvqiGXaHKoSWMgK7xUdV7mdgsB4HiLfhsqqr6B3c3nzdyCvwFNFN8d1ZWe436yELa
Or should the key come from the Mnemonic entrophy string (also newly created & empty)?
e0b21dfb18343ead2254061b20eac319
From BIP39:
The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
Essentially, any random string of 128-256 bits can be converted into a BIP39 mnemonic.
However, if you're asking whether you can calculate a BIP39 mnemonic that would generate a given private key, that's not possible regardless of wallet scheme used because BIP39 specifies that to seed a wallet you need to run your mnemonic through a cryptographic hash function first.
