Moreno Garciaon Stack Overflow • Recommends
On “Scylla datacenter and Cassandra datacenter in same cluster”
You can not have an heterogeneous cluster with C* and Scylla nodes on the same cluster.
Create a separate scylla cluster, create the schema, change the app to do double writes (to both clusters) and then migrate the C* historical data to Scylla.
There are multiple ways to migrate the data. This should help: link to this video No downtime, no problem there are options for that too.