This paper presents a specific approach to Business Process design by combining selected principles of Adaptive Case Management, traditional modeling of processes executable in Business Process Management Systems, and a constraint-based approach to process design. This combined approach is intended for business situations, where traditional process models with rigid structures can lead to limitations of business flexibility. We propose a process design pattern that is suitable for the modeling of ad-hoc processes within common BPMS-based systems. The pattern can be used to define a process structure in a declarative constraint-based manner. Further, we present an application of the approach in an actual project, which is an end-to-end BPM project from an insurance business. The project uncovered needs for an extended flexibility of process structures. This along with requirements based on ad-hoc processes led to advancement in the presented approach. This paper presents a versatile, generally applicable solution, which was later tailored for the purpose of the aforementioned project and led to the successful satisfaction of the requirements. The approach is part of a more comprehensive research effort – complex BPM adoption methodology BPM4SME designed primarily for Small and Medium Enterprises, which put emphasis on the agility of the BPM adoption process and consequent flexible implementations of BPMS-based systems.