That shouldn't be a problem. The Custom Constituent Import (under "Data Management" > "Import/Export") allows the upload of email addresses without any other identifying information. That upload also produces groups that contain the new and modified records affected by the sync you ran. To find those groups:
  1. Go to Constituent360 > Groups.
  2. On the left expand "All Groups".
  3. Find and click on "Data Sync" under "All Groups".
  4. Search for the date of the sync in the format of YYYY/MM/DD.
This should bring up one or two groups that look like this:
  • "DS - 2013/09/24 11:32:00 - Mod" - This contains any existing records that were modified by the sync.
  • "DS - 2013/09/24 14:35:47 - New" - This contains any new records that were created by the sync.