New cropping systems are needed to meet the increasing demand for sustainable feedstock for food, feed, fibre, and fuel. However, the lack of agronomic knowledge has been shown to hinder the introduction of new crops in current cropping systems. In design sciences, the C-K theory, which was developed to model design logics, shows that a design process could be a relevant tool to produce knowledge. In this study, an original participatory design approach combining a multi-actor workshop and on-farm trials was developed to produce actionable knowledge and identify knowledge gaps. The work was based on a case study: the introduction, in the cropping systems of northern France, of a little-known oilseed crop, camelina [Camelina sativa (L.) Crantz], to supply a local oilseed biorefinery. During the workshop, farmers, researchers, and industrialists collectively designed eight crop sequences that included camelina as a second crop, and two crop management options, as a relay crop and a double crop. These design activities allowed for the identification of: (i) assessment indicators derived from participants' expectations; and (ii) knowledge gaps about camelina crop functioning, management, and preceding crop effects. After the workshop, four farmers implemented on-farm trials. They autonomously designed and managed thirteen crop management options, and appraised them according to their own indicators. These on-farm trials were monitored through interviews, field tours, and measurements. Eleven criteria used by the farmers to appraise their trials were identified. Four crop management options were considered as conclusive, three as not conclusive, and six as promising. The monitoring of these original on-farm trials allowed for: (i) the identification of farmers' monitoring indicators; (ii) the formulation of decision rules for camelina management; (iii) a better understanding of camelina crop functioning; and (iv) the identification of farmers' questions, revealing knowledge gaps to be fulfilled to enhance camelina areas. In this study, we demonstrated that participatory design could be used to: (i) support, at a low cost, the production of actionable knowledge on a little-known crop; and (ii) pursue the identification of relevant topics lines for research-action programs. We also showed that the combination of a multi-actor workshop and new kinds of on-farm trials is relevant to reach these objectives. In conclusion, this approach could be an interesting way to support the design of diversified cropping systems, including little-known crops such as camelina, and to help in the setting of research priorities for these crops.