If you started a checklist run, but noticed there is a step missing, you can update the checklist template and that will add the missing step to the checklist run. This works if you're adding steps, changing title/description, removing steps, changing the order, whatever.
On the checklist run settings tab, there is a link to edit the template.
1. Checklist Run - Settings Tab
Then, after you make changes to the template a button will appear for you to update the in-progress checklist runs. Click that to update the checklist run with these changes to the template.
2. Checklist Template - Editing Page