XStream - Atributo de XML pode ser uma Classe Java ou apenas valor literal?

public class Test {

	public static void main(String[] args) {
		String x = "<proposicao Ano='2010' Numero='8050' Tipo='PL' data='1154097812245'><idProposicao>490340</idProposicao></proposicao>";

		XStream xstream1 = new XStream();
		xstream1.processAnnotations(ProposicaoWebServiceTO.class);
		xstream1.registerConverter(new Test().new TipoConverter());
		xstream1.registerConverter(new SingleValueCalendarConverter());
		System.out.println(xstream1.fromXML(x));
	}

	@XStreamAlias("proposicao")
	class ProposicaoWebServiceTO {

		@XStreamAlias("idProposicao")
		private Long id;

		@XStreamAsAttribute
		@XStreamAlias("Tipo")
		private TipoProposicaoTO tipoProposicao;

		@XStreamAsAttribute
		@XStreamAlias("Numero")
		private int numero;
		
		@XStreamAsAttribute
		private GregorianCalendar data = new GregorianCalendar();
		
	}
	
	

	@Data
	class TipoProposicaoTO {
		private String siglaProposicao;
	}
}

class TipoConverter implements SingleValueConverter {

		public String toString(Object obj) {
			return ((TipoProposicaoTO) obj).getSiglaProposicao();
		}

		public Object fromString(String name) {
			TipoProposicaoTO tipoProposicaoTO = new TipoProposicaoTO();
			tipoProposicaoTO.setSiglaProposicao(name);
			return tipoProposicaoTO;
		}

		public boolean canConvert(Class type) {
			return type.equals(TipoProposicaoTO.class);
		}

	}