Please answer the following question:-
1) Cluster and Database binaries version?
2) Is single instance and cluster database binaries version are same or different?
3) Is your database running on different server or running on one of RAC node server?
Please refer to the following link which demonstrates migration of a RAC database from one cluster to another.
You can follow the same steps even for migrating a single instance database to a RAC environment.
Hope it helps.
Better to follow the docs as suggested. In this doc DBCA is used to convert database from single instance to cluster database. I just summarize the steps mention in the docs which will help you probably while reading the docs.
Step 1> Invoke DBCA where single instance database is running and create a database template with Structure as well as Data option.
Step 2> Move Template files from single Instance server to any one of the RAC node server.
Step 3> Invoke DBCA from the node where you copied the template files and create a RAC database using that template.