fracking derives directly from fracture which is English and follows normal practice of modifying any derivative so that it resembles the source word in sound.
And which is pronounced frack-ture not frash-ture.
I might also mention beacon, deacon, meaconing, pecan, pica, picaroon, racoon, recall, recant, recap, recast, ricochet, rictus (another old Latin word), roc, sac, sacerdotal, seclude, second, secret...... etc
So there is no need to add a K to make a hard C, there are numerous other examples in the gloriously inconsistent English language.