Checklists and action plans are a proven mechanism for project-based collaboration. Synthesizing project-specific plans is challenging, as project managers must consider multiple sources of information, from structured surveys to semi-structured conversations with stakeholders. In a needfinding study with project managers, we identified challenges in creating action plans for teams. We built MixTAPE, a mixed-initiative system that addressed these challenges with three components: a semi-structured note-taking interface for capturing stakeholder conversations, a plan generator for automatically combining multi-source information into action plans, and classification models for assigning and prioritizing action items. We evaluated MixTAPE in an observational study of 32 website design projects. Compared to a previously unstructured process, MixTAPE generated 1.45X as many tasks that are more consistent, while reducing the plan creation time by 33.70%. Through interviews and surveys, we found that participants rate MixTAPE highly across several measures. Based on our findings, we discuss the implications and opportunities for mixed-initiative action plan creation.