There are three scenarios in which a single-instance database can exist on a cluster node:
Scenario 1: The Oracle home for the single-instance database was installed on a cluster node and has Oracle RAC enabled. The steps to perform a conversion to Oracle RAC in this scenario are described in "Single-Instance Database on a Cluster Running from an Oracle RAC-Enabled Home"
Scenario 2: The Oracle home for the single-instance database was installed on a cluster node, but the Oracle RAC feature is disabled for this Oracle home. Refer to "Single-Instance Database on a Cluster Running from an Oracle RAC-Disabled Home" for instructions on how to convert the database to Oracle RAC in this environment.
Scenario 3: The Oracle home for the single-instance database was installed on only the local node in a cluster. This happens when you select the Local Installation option on the Oracle Universal Installer Specify Hardware Cluster Installation page during the Oracle Database 12c installation. The procedure to convert the single-instance database to Oracle RAC in this scenario is the same as that described in "Converting Oracle Database Installations to Oracle RAC Using DBCA" with one exception. In the last step, "Install Oracle Database 12c Software with Oracle RAC" ensure that you select a new Oracle home, not the one from which the single-instance database is running.