Is it possible to convert an X/Y/Z-PUB Key without a seed extension to what the X/Y/Z-Pub Key would be for the same seed with a Mnemonic added?
This is only possible through bruteforcing the 25th word to the (known) mnemonic seed.
In other words, without access to the seed, can a Master Public Key be converted to what said Master Public Key would be if a (known) passphrase were added to the seed without knowing the seed itself?
No. Without access to the seed, you can not create a xpub at all.
As far as i understand you, you are worried about the security of the 25th word ?
As long as your seed is kept secret, noone will gain information about your xpub/xpriv/addresses/etc.. The xpub of your seed without the 25th word doesn't make a difference here.