A cryptosystem that allows inferring the secret key (necessary for encryption/decryption) from plaintext+ciphertext with less-than-brute-force effort is considered broken. I'm guessing that successful cryptolockers use non-broken encryption. So no, having a plain and encrypted version of the same file is not enough to undo cryptolocker damage.
Usually symmetric ciphers use a block size between 128 and 256 bits, the amount of different blocks you can compose from those (and which your file would need to contain) is astronomical. Also, when used properly, the cipher output is also dependent on the cipher's internal state (initialized with a random vector, likely different for each file), so that repeated encryptions of the same block of plaintext in a different file (or different position in the same file) will be different.
Not a specialist, but I took the crypto 101 at school.