In Swagger-Net you have SwaggerResponse you can call it like these:
[SwaggerResponse(400, "Bad request")]
public class SwaggerAnnotatedController : ApiController
{
    [SwaggerResponseRemoveDefaults]
    [SwaggerResponse(HttpStatusCode.Created, Type = typeof(int), MediaType = "text", Examples = 123)]
    [SwaggerResponse(HttpStatusCode.BadRequest, "Invalid message", typeof(HttpError))]
    public int Create(Message message)
    {
        throw new NotImplementedException();
    }
    [SwaggerResponse( 200, Type = typeof( IEnumerable<Message> ), TypeName = "Messages" )]
    public IEnumerable<Message> GetAll()
    {
        throw new NotImplementedException();
    }
But I never had a need for xmlnamespace so there is no such an option: 
https://github.com/heldersepu/Swagger-Net/blob/master/Swagger.Net/Swagger/Annotations/SwaggerResponseAttribute.cs
but if you can provide precise details of how is that xmlnamespace used I can add it.