Similar to right-clicking an email to Create Rule from Message. The ability to add an email to an existing rule would be a good feature.
To add another email address to an existing rule, Edit the Rule and then “Click on an existing email address already there” which will then bring up a window with the current email address list, where you can then add another email address.
(eM Client V10 Add email address to current rule example)
Thanks I know that. Feature request was asking for a simpler/faster way to do this, i.e. right-click and add.