Emotion regulation (ER) is a dynamic process that often unfolds in social contexts. However, current digital ER tools predominantly rely on single-agent systems that lack the complexity of social dynamics. Swarm user interfaces present unique affordances for ER through their collective adaptability and expressive group behaviour. However, their potential in supporting ER remains underexplored. To investigate how swarm user interfaces can be designed to support ER, we conducted a series of speculative participatory design workshops with 15 participants through the Magic Machine Workshop method, where participants created and enacted interactive swarm-based artifacts with craft materials. The analysis led to diverse contexts of use, envisioned swarm framing, and interaction modes. Based on these findings, we synthesize eight interaction patterns that translate abstract user metaphors into robotic behaviors. We conclude by articulating design opportunities and challenges, positioning swarm interfaces as a novel medium for ER support.