You can use your existing Opened Email smart campaign that adds leads to your nurture campaign. You will need to add the Campaign is Requested trigger to it, so that you can run it on the list of leads you want to add to it. Then you would create a smart list with the Opened Email filter and specify the data to select the leads that haven't yet been added to the nurture campaign. Then select all the leads in the list, chose Lead Actions | Special | Request Campaign, enter the name of your Opened Email smart campaign and click Run Now.