Like lite said above, it is impossible.
The process to generate a BIP 39 mnemonic seed actually generates
entropy and encodes it
then a checksum is added And it is then mapped to a word list.
So it is impossible to go from a a mnemonic seed back to entropy.
The BIP 39 ---> (WIF) private key is a one-way deterministic function and you can't reverse it
However, since you have the WIF (Wallet
import format) private key then you can import it directly into a wallet that supports that function -- lots of wallets do.
If, for whatever reason, you lost your seed words but just have the private key for one of the generated addresses (for a BIP 32 generated HD address), then unfortunately you cannot use it to get the remaining (change) addresses.