Rolling over leave balances from one year to the next is done with a CSV file import. (This is the same step you would have taken when first importing balances into Workforce.com.)
Import Leave balances
Navigate to Leave > Requests > Tools > Leave Balances, then follow the steps listed on that page.
Staff that accrue leave generally start with 0 hours. However, if you are rolling over leave balances, you will need to put in the amount of hours you are rolling over in the cell that pertains to that employee. From there you just need to save and re-upload.