Design tools for personal fabrication or mass customization enable users to design personalized products that fit their needs and desires. Advanced technologies such as Artificial Intelligence (AI) or simulation have opened up new possibilities for digital design systems to support non-professionals. Still, most of the existing works have been focused on translating the users’ needs into a form of product; and how to provide expert support within the design system throughout the product customization process is still missing. In this paper, we look into how digital design systems could take the role of expert designers. We conducted a group interview with four expert product designers, each having over ten-year experience in designing and realizing personalized product solutions for individual users. By collecting and analyzing how they collaborate with and assist users throughout the product customization process, we identify four types of expert roles: conflict negotiation, knowledge support, solution suggestion, and real-time feedback. Based on the findings, we discuss design opportunities for expert support for interactive design systems.