Our recommendation to import any Transactions that existed before your Integration was installed would be to first contact Sisu and provide a CSV file of Closed (pre-Realsynch) Transactions. Sisu can then perform a file import for those Closed Transactions.
Next, Activate Your Integration(s).
Finally, make trivial updates to any Deals or Transactions in your source-system that are in a status prior to Closed to trigger the integration to execute to create these Transactions in Sisu. Additional updates made to a given Transaction in your CRM/TMS will update the related Sisu Transaction moving forward.