Your existing accruals will sync automatically from your accounting file. Once you have activated accruals simply re-sync your Mayday group and you will find all your existing accruals in the summary and schedule!
They can be easily identified by the relevant accounting file icon on the left side:
Note: for existing accruals you are not able to edit any of the details in Mayday, that were created outside of Mayday. But if you haven’t already reversed the accrual, you can do that via Mayday and you can also assign a contact to the accrual. See this article for more information on reversing accruals.