When new customers join Engager, they often ask whether they can Engager to sync up their existing Client References Numbers / IDs to Engager, and Engager to follow suit sequentially.
Unfortunately this is not possible and a lot of it comes down to audit purposes and how Engager logs the information.
What you can do though, is create a Custom Field for your Existing 'Client IDs / references' so that you have these within Engager. Engager will assign 1,2,3,4 sequentially for your clients, but you'll at least have your previous system.