Software product lines have proved to be a successful and efficient means for managing the development of software in industry. The significant benefits over traditional software architectures have the potential to convince software companies to adopt the product line approach for their existing products. In that case, the question arises how to best convert the existing products into a software product line. For several reasons, an evolutionary approach is desirable. But so far, there is little guidance on the evolutionary introduction of software product lines. In this paper, we propose a lightweight iterative process supporting the incremental introduction of product line concepts for existing software products. Starting with the analysis of the legacy code, we assess what parts of the software can be restructured for product line needs at reasonable costs. For the analysis of the products, we use feature analysis, a reengineering technique tailored to the specific needs of the initiation of software product lines.