How to deal with Carbon Copy Cloner backing up various OS versions of a given machine
Based on the info in your question you should be able to restore an appropriate backup image to another external drive. 

Once you've done this, boot your Mac into the Startup Disk Manager (i.e. press and hold the option key as you boot up) until you see the drive options appear. Select the disk you've just restored the Snow Leopard backup to and press enter to see if you can boot from it.

If it boots okay, then you can try to convert the files you want to use in macOS Sierra.

NOTES:

1 If your drive doesn't appear in the Startup Disk Manager, there are a number of possible reasons for this. If this happens to you, then while you're in the Startup Disk Manager you can try pressing the option+shift+command+. keys to see if your external drive has optional ROM firmware available.

2 If you can't boot up from the drive, refer to the Help! My clone won't boot! page for a detailed list of possible causes/solutions.