I am not aware of existing utilities for deserialization, but as Marc has suggested, you might want to see if there are old Titan resources regarding it, since the data model hasn’t been changed since Titan -> JanusGraph migration.
If you want to resort to the source code, you could check out EdgeSerializer and IndexSerializer. Here is a simple code snippet demonstrating how to deserialize an edge:
final Entry colVal = StaticArrayEntry.of(StaticArrayBuffer.of(Bytes.fromHexString("0x70a0802140803800")), StaticArrayBuffer.of(Bytes.fromHexString("0x0180a076616c75e5"))); // I retrieved this hex string from Cassandra cqlsh console
final StandardSerializer serializer = new StandardSerializer();
final EdgeSerializer edgeSerializer = new EdgeSerializer(serializer);
RelationCache edgeCache = edgeSerializer.readRelation(colVal, false, (StandardJanusGraphTx) tx); // this is the deserialized edge
Bytes.fromHexString is an utility method provided by datastax cassandra driver. You might use any other library/code to convert hex string to bytes.
As you can see, there is no single easy-to-use API to deserialize raw data. If you end up creating one, I think it would be helpful if you could contribute back to the community.
There should be some old Titan resources that describe how the data model is binary coded into the row keys and row values. Of course, it is also implicit from the JanusGraph source code.
If you look back at this week's OLAP presentations (https://lists.lfaidata.foundation/g/janusgraph-users/topic/janusgraph_meetup_4/82939376)
you will see that one of the presenters exactly did what you propose: they exported rows from scylladb and converted it to gryo format for import into TinkerPop HadoopGraph. You might want to contact them to coordinate a possible contribution to the JanusGraph project.
Best wishes, Marc