Shape grammars provide a concise and computable framework for design space exploration. An important aspect of design space exploration, especially in the case of grammar-based exploration of visual languages, is the ability to modify grammar rules in order to reach design solutions that are not in the scope of the initial grammar. In this paper, we describe and implement a semi-automatic approach for the 'on-the-fly' generation of new rules. In particular, new rules are added to a shape grammar by manually modifying the current shape in the derivation. This approach might enable a more agile exploration of the design space. Also, we demonstrate a proof-of-concept of the proposed approach based on an existing graph-theoretic framework for the computer implementation of shape grammars.