Yes, you can. Depending on whether your pre-populated folders are located on linked devices or on devices with different Sync identities there's a special workflow. When peers connect to each other, Sync will rescan the files, hash them and compare
- same files with same hashes will not be re-synced;
- if same files have different hash (for example, different content) the one with latest timestamp will be synced replacing the file on remote peer;
- other files will be just merged into folder tree and exchanged.
The following instruction works for mobiles and step will be basically the same, though the UI views will be different from the examples below.
On devices with different identities:
Device 1 - Source
Choose a folder you wish to synchronise and add it to Sync, click on "Share" button, copy the link or key and deliver it to Computer 2:
Device 2 - Destination
If device2 is an Android mobile, disable Simple mode in settings. More details here.
Take the key/link delivered from Computer 1 and paste it to "Manual connection" field in menu "+" > "Enter a key or link". When prompted for folder location, point it to the already existing directory and click "Connect".
If the folder's not empty, you'll see a message asking for confirmation. Click 'OK' to connect two folders.
On linked devices:
Before starting, go to "My devices" -> This Device and switch the mode to Disconnected. Alternatively, you can keep it in Connected or Synced, however, once the added folder appears on Computer 2, you should disconnect it.
On mobile device disable Simple mode in settings. More details here.
If the folder's not empty, you'll see a message asking for confirmation. Click 'OK' to connect two folders.
Note:
When using mobile's backup with linked devices, the share will always appear connected on destination desktops regardless of the syncing mode. You need to disconnect the backup share and reconnect it.