A user can define their own ConversionService bean and spring reuses it (example 1, example 2) if the bean name is conversionService and it's of the org.springframework.core.convert.ConversionService type.

Unfortunately spring-data-cassandra always creates a new org.springframework.core.convert.support.DefaultConversionService in MappingCassandraConverter#newConversionService which prevents users from tweaking the conversionService the way they need.