To utilize preseeding, you'll need to first start the process of creating a VPG. Once you have selected the VM's to be protected, and chosen the recovery site resources, you'll need to select the storage settings:
From this screen, you'll want to check off the volume to preseed, and select 'Edit Selected'. You'll then be presented with the 'Edit Volume' dialog. In the 'Volume Source' dropdown, select 'Preseeded Volume':
Once selected, you'll be presented with the options to select the datastore and path of the preseeded disk. Use the 'Datastore' dropdown to select the datastore which currently holds the copied data, and select the "..." button to bring up the "Browse for file" dialog:
Once you have the browser open, browse to the path containing your preseeded data. You will only be able to select VMDK's which exactly match the size of the protected volume:
Once you have selected the appropriate VMDK, click "Save". Repeat this process for all volumes for which you have pre-existing copies. All volumes which are not preseeded will be created by Zerto. Continue through VPG creation as normal. When you have finished creating the VPG, if all volumes were preseeded, the VPG status should be in "Delta Sync" instead of the normal "Initial Sync"