So, you have a new server for your planned cluster to add to the current machine with the stand alone instance.

Do you have a maintenance window or agreed downtime period in which to perform this move?

I would build the new cluster on the new single node primarily with new storage presented. Get the instance up and running and move across any system objects such as logins, jobs, etc.

Also get across as many user databases as possible then point the applications connection strings to the new clustered instance.

If you're wanting to use the existing servername you'll need to shut the server down before you can use the servername as the VNN on the new clustered instance.