Create inheritable LINQ-to-SQL mapable class

I am building a library for Windows Phone 8 which requires local databases. For obvious reasons, user of the library is going to create a mappable LINQ-to-SQL class with appropriate [Table]s and [Column]s. However, To in every such class, I need a few more columns for internal functioning of the library. The idea was that I would include a base class in the library which will have members corresponding to the required columns. The user would simply inherit from this class, add his own members and use that class as final LINQ-to-SQL map.

So far, my base class looks like this:

//Class to contain all the essential members
[Table]
public class SyncableEntityBase : NotifyBase, ISyncableBase
{
    [Column(DbType = "INT NOT NULL IDENTITY", IsDbGenerated = true, IsPrimaryKey = true)]
    public int ItemId { get; set; }

    [Column]
    public bool IsDeleted { get; set; }

    [Column]
    public DateTime RemoteLastUpdated { get; set; }

    [Column]
    public DateTime LocalLastUpdated { get; set; }       
}

And the derived class, something like this:

[Table]
public class ToDoCategory : SyncableEntityBase
{
    private string _categoryName;
    [Column]
    public string CategoryName
    {
        get
        {
            return _categoryName;
        }
        set
        {
            if (_categoryName != value)
            {
                NotifyPropertyChanging("CategoryName");
                _categoryName = value;
                NotifyPropertyChanged("CategoryName");
            }
        }
    }

    private string _categoryColor;
    [Column]
    public string CategoryColor
    {
        get
        {
            return _categoryColor;
        }
        set
        {
            if (_categoryColor != value)
            {
                NotifyPropertyChanging("CategoryColor");
                _categoryColor = value;
                NotifyPropertyChanged("CategoryColor");
            }
        }
    }
}

Idea is to have final class with the four essential columns and two added by user. According to MSDN documentation here, I need to append [InheritanceMapping] which requires the inherited type. However, as I am building a library, I have no way to know what types (and how many) the user will derive from my base class. Is there any way around this? How?