That isn't possible since the way bip39 works the seed is passed through hashing functions and out comes the master private key. By definition the output of a hashing function cannot be reversed to get the input of the function. So it's a one way process.
However it should be possible to take the extended private key and encode it as a mnemonic using some other algorithm i.e. not bip39.
You could convert a number to a mnemonic with the old electrum algorithm. Pre 2.0. See under mnemonic here:
https://bitcoinspakistan.com/blog/electrum-seed-explained/
However an extended private key is more than just one number. There is a chain code, a key, depth and version number. So lots of stuff in there. Somebody would have to unravel all that.