How to add field not mapped to table in Linq to Sql

In Entity Framework I can apply NotMapped attribute to a property which I do NOT want to create a column in a database table for. How to get the same effect for auto generated classes in DBML file? I have a StoredProcedure that returns some additional fields. I called SP like:

[global::System.Data.Linq.Mapping.FunctionAttribute(Name = "dbo.sp_GetSupplierArticles")]
public ISingleResult<SupplierArticle> GetSupplierArticles([global::System.Data.Linq.Mapping.ParameterAttribute(DbType = "BigInt")] long mainArticleId, [global::System.Data.Linq.Mapping.ParameterAttribute(DbType = "BigInt")] long? userId)
{
    IExecuteResult result = this.ExecuteMethodCall(this, ((MethodInfo)(MethodInfo.GetCurrentMethod())), mainArticleId, userId);
    return ((ISingleResult<SupplierArticle>)(result.ReturnValue));
}

Necessary field I added into the separated partial class. Without any additional attributes it returns default value for my and applied [Column(IsDbGenerated = false)] in the separated partial class:

public partial class SupplierArticle
{
    [Column(IsDbGenerated = false)]
    public double Extra { get; set; }
}

So it works until I try to get SupplierArticle using another query (not my stored procedure):

db.LoadOptions = db.GenerateDataLoadOptions(entitiesToInclude);
var query =
    from shoppingCartItem in db.ShoppingCartItems
    where shoppingCartItem.UserId == userId
    select shoppingCartItem;
return query.ToList();

My entity is loaded due to LoadOptions (passed in entitiesToInclude parameter). In this query and another which try to load "poor" entity with properties that defined in .dbml file I get exception: Invalid column name 'Extra' and the same message for each additional property.

What is the proper way to extend entity or how to avoid that exception?

UPD: If I remove all attributes exception no longer occurs. But added properties are not initialized when SP returns a result.