There are a several ways to make this happen.  You mentioned Workflow so I assume you do not want to create a standalone utility using the SDK to do the search and export so I will stick with the Workflow only options. 

You will need to use the SDK Script activity inside of a For Each Entry activity.  The Entries source for the For Each Entry will be the Output Entries from the Search Repository activity that you already have configured.  The actual scripting of the SDK Script activity will be up to you but there are several code examples on the Answers website.  Here is one https://answers.laserfiche.com/questions/118913/sdk-export#127917  but you can do a search for 'export script' and get several more hits.  Note:  You do not need the actual SDK installed to use the SDK Script workflow activity but you can use the documentation from the SDK to help with your coding. 

The other option if you don't want to code it up yourself is to use the DocumentExport custom workflow activity from Qfiche at http://qfiche.com/products.  The downloads available from the Qfiche website are fully functional 30-day demo activities.  Once installed the custom workflow activities behave exactly like native workflow activities and expose all of the properties that you need to use the activity.