This article introduces a novel approach to the problem of collaborative planning. We present a method that takes classical one-shot planning techniques - that take a fixed set of goals, initial state, and a domain theory - and adapts them to support the incremental, hierarchical and exploratory nature of collaborative planning that occurs between human planners, and that multi-agent planning systems attempt to support. This approach is planner-independent - in that it could be applied to any classical planning technique - and recasts the problem of collaborative planning as a search through a space of possible inputs to a classical planning system. This article outlines the technique and describes its application to the Mission Rehearsal Exercise, a multi-agent training system.