I am afraid that this is not possible. As far as I know Cassandra is a non-relational SQL Database. For this we do not have any sensors available as any non-relational SQL Database works differently. We also have little to no requests for a sensor to monitor a Cassandra Database Server.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.