scratch the hard surface , and wet with water - do this several times, till the knife ( very THIN blade, !) penetrates the clay easily 4 mm deep .
Only then has a "bonding surface" been created, to which the new clay can attach.
if the hard clay is left hard, and set call attached: the new clay will attach initially, but come off when dried
It is of advantage to let the new clay NOT be too wet - the greater the difference in dry/wetness, the more likely crack s appear later, due to the SHRINKAGE of the wet clay
if the clay is very hard, the knife will hardly penetrate - don't force it , if the cutting destroys the clay,the pressure is too hard.
better to do it in seveeral goes, let the water soften the clay, to help the knife penetrate deeper into the clay surface