Converting existing media servers to cluster media servers

No. Existing standalone media servers cannot be installed in active/passive clustered configuration.
NO media server can be  active/passive clustered media server. 
Only master servers can be clustered this way.

Leave standalone media server installation.

As per my previous post, your choices are Storage Unit Groups or where onther applications are already clustered, configure Application Cluster.

PLEASE go through  NetBackup in Highly Available Environments Administrator's Guide  for detailed info.

Start on p.17:

About protecting against dedicated media server failures
Storage unit groups can be used to protect NetBackup against the failure of a single
media server. Storage unit groups can also be used for load balancing across
multiple media servers to ensure optimal backup and restore performance.
Table 2-4 discusses the different modes in which you can configure the storage
unit groups.

(more in the manual)

Then go to p. 24:

About installing NetBackup media software in a cluster
Install the NetBackup media server software on each node in the cluster. See the
NetBackup Installation Guide for details.

Note: When you configure NetBackup as described in this topic, do not configure
the NetBackup media server as a failover application. The media server must be
treated as a stand-alone application on each node in the cluster.

(more in the manual)