Spark Kryo: Register a custom serializer

I have a class that implements a custom Kryo serializer by implementing the read() and write() methods from com.esotericsoftware.kryo.Serializer (see example below). How can I register this custom serializer with Spark?

Here is a pseudo-code example of what I have:

class A() 

CustomASerializer extends com.esotericsoftware.kryo.Serializer[A]{
    override def write(kryo: Kryo, output: Output, a: A): Unit = ???
    override def read(kryo: Kryo, input: Input, t: Class[A]): A = ???
}

val kryo: Kryo = ... 
kryo.register(classOf[A], new CustomASerializer()); // I can register my serializer

Now in Spark:

val sparkConf = new SparkConf()
sparkConf.registerKryoClasses(Array(classOf[A]))

Unfortunately, Spark doesn't give me the option to register my custom serializer. Any idea if there is a way to do this?