[Context and motivation] Requirements engineers and stakeholders like to create informal, sketchy models in order to communicate ideas and to make them persistent. They prefer pen and paper over current software modeling tools, because the former allow for any kind of sketches and do not break the creative flow. [Question/problem] To facilitate requirements management, engineers then need to manually transform the sketches into more formal models of requirements. This is a tedious, time-consuming task. Furthermore, there is a risk that the original intentions of the sketched models and informal annotations get lost in the transition. [Principal ideas/results] We present the idea for a seamless, tool-supported transition from informal, sketchy drafts to more formal models such as UML diagrams. Our approach uses an existing sketch recognizer together with a dynamic library of modeling symbols. This library can be augmented and modified by the user anytime during the sketching/modeling process. Thus, an engineer can start sketching without any restrictions, and can add both syntax and semantics later. Or the engineer can define a domain-specific modeling language with any degree of formality and adapt it on the fly. [Contribution] In this paper we describe how our approach combines the advantages of modeling with the freedom and ease of sketching in a way other modeling tools cannot provide.