Yes, version sets can be applied to existing mailings. To do this, follow these steps:  
> Go to the mailing you want to add the versions to and click into it so  you can see your master version  
> Click on settings (next to the red + add  new mailing button)  
> Select version sets  
> Select the version set you want from the drop down and click 'Add  versions from set'  When you click into your mailing, all the versions will now be created.

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