Ищи в MSDN
<webServices> Element

Ну дык так и писать
public НужнаяСтруктура GetListCategories(...){...}
и добавить XmlIncludeAttribute
Наследовать, если нужно.

using System;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Xml.Serialization;
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class Service : System.Web.Services.WebService
{
    public Service ()
    {    }
    [WebMethod]
    [XmlInclude(typeof(BlondyWorld))]
    public World HelloWorld()
    {
        return(new BlondyWorld());
    }
}
public class World
{
    private int size;
    public int Size
    {
        get { return this.size; }
        set { this.size = value; }
    }
}
public class DumbWorld : World
{
    private string myName;
    public string MyName
    {
        get { return "Dumb"; }
        set { this.myName = value; }
    }
}
public class BlondyWorld : DumbWorld
{
    private Int64 preferedSize;
    public Int64 PreferedSize
    {
        get { return Int64.MaxValue; }
        set { this.preferedSize = value; }
    }
}