I see that Sorm already supports org.joda.time.DateTime. Is there a possibility to add support for other types?
For example, my case class has a java.nio.charset.Charset or Locale field, which I would like to convert to a string. Suppose I have functions to accomplish the conversion from the custom type to/from a SQL type, how can I tell Sorm to use it?
SORM's support for a certain datatype is quite more complex than just ability to convert to and from an SQL type. Values of some types may span several columns (e.g. Tuple, Range), others may require intermediate tables (Seq, Set, Map) and all of them require an individual approach to translating query clauses. All that would have resulted in a quite complex ad-hoc type-mapping API if one was to be exposed.
But the thing is the above is really not the reason why such an API is not exposed and most probably not to ever be. You see, SORM's philosophy is essentially all about pure immutable data model and the cleanest way to design such one is to use standard Scala's immutable datatypes and case classes.
So the clean way for you to design your application with SORM would be to convert those stateful Java's classes to immutable values in your application. For instance you could implement a custom case class Charset (...) in your model, register it with SORM's Instance and have your conversion functions work between this type and the Java's one in your application. Besides that, you could implement this Charset as an Enumeration, which seems to be the most appropriate.
Concerning your argument about the Joda Time types support, it's there mostly because some data types were needed to represent the SQL's timestamps. See this logic as reverse to what you were thinking of.