Interactive end-user training of machine learning systems has received significant attention as a tool for personalizing recognizers. However, most research limits end users to training a fixed set of application-defined concepts. This paper considers additional challenges that arise in end-user support for defining the number and nature of concepts that a system must learn to recognize. We develop BeatBox, a new system that enables end-user creation of custom beatbox recognizers and interactive adaptation of recognizers to an end user's technique, environment, and musical goals. BeatBox proposes rapid end-user exploration of variations in the number and nature of learned concepts, and provides end users with feedback on the reliability of recognizers learned for different potential combinations of percussive vocalizations. In a preliminary evaluation, we observed that end users were able to quickly create usable classifiers, that they explored different combinations of concepts to test alternative vocalizations and to refine classifiers for new musical contexts, and that learnability feedback was often helpful in alerting them to potential difficulties with a desired learning concept.