This paper addresses the question of how to extend the capabilities of a well-established interface in a way that respects users' existing expertise. The piano-style keyboard is among the most widely used and versatile of digital musical interfaces. However, it lacks the ability to alter the pitch of a note after it has been played, a limitation which prevents the performer from executing common expressive techniques including vibrato and pitch bending. We present a system for controlling pitch from the keyboard surface using capacitive touch sensors to measure the locations of the player's fingers on the keys. The large community of trained pianists makes the keyboard a compelling target for augmentation, but it also poses a challenge: how can a musical interface be extended while making use of the existing techniques performers have spent thousands of hours learning? In this paper, user studies with conservatory pianists explore the constraints of traditional keyboard technique and evaluate the usability of the continuous pitch control system. The paper also discusses implications for the extension of other established interfaces in musical and non-musical contexts.