In order to replace the create URL segment code you need to inherit from EPiServer.Global and override the EPiServer.Global.Application_Start(Object, EventArgs) and EPiServer.Global.GlobalPageValidation_Validators(object, PageValidateEventArgs). The following code shows an example of how to replace the default functionality with custom code.
public class Global : EPiServer.Global { protected void Application_Start(Object sender, EventArgs e) { GlobalPageValidation.Validators -= EPiServer.Web.UrlSegment.ValidateEventHandler; GlobalPageValidation.Validators += new PageValidateEventHandler(GlobalPageValidation_Validators); } void GlobalPageValidation_Validators(object sender, PageValidateEventArgs e) { e.Page["PageUrlSegment"] = "JustTesting"; //Apply your own logic here } //... //... //... }