How is Structure storing the data in issues? Is it at all? We have been doing reporting based on Epics that Portfolio uses for similar purpose (I think), but not sure how Structure does it. In essence either you can use the custom fields in Jira import to define new dimensions (e.g. if the board name is stored on the issue itself in a custom field) or you can try to use Excel to add new dimensions based on links between issues and the boards.