I do not see any way to bulk add checklist text or checklist JSON to items already created. There is an option to Change Checklist YAML but posting the JSON there doesnt work
The "Add Checklists to Jira Issues / Tickets in Bulk" section on this page seems obsolete: https://community.atlassian.com/forums/App-Central-articles/10-Ways-to-Add-Checklists-to-Jira-Issues/ba-p/2936450
