I have know idea if this will work...

Hierarchyize.cs

    public static class Hierarchyize
    {
        public static List<Nested<T>> Transform<T>(IEnumerable<T> items, Func<T, T, bool> isParentFunction, Func<T, bool> isRootFunction)
        {
            Action<Nested<T>> setChildren = null;

            setChildren = parent =>
            {
                parent.Children = items
                    .Where(childItem => isParentFunction(childItem, parent.Item))
                    .Select(childItem => new Nested<T> { Item = childItem })
                    .ToList();

                //Recursively call the SetChildren method for each child.
                parent.Children
                    .ForEach(setChildren);
            };

            //Initialize the hierarchical list to root level items
            List<Nested<T>> hierarchicalItems = items
                .Where(rootItem => isRootFunction(rootItem))
                .Select(childItem => new Nested<T> { Item = childItem })
                .ToList();

            //Call the SetChildren method to set the children on each root level item.
            hierarchicalItems.ForEach(setChildren);

            return hierarchicalItems;
        }
    }

Nested.cs

    public class Nested<T>
    {
        public T Item { get; set; }
        public List<Nested<T>> Children { get; set; }

        public Nested()
        {
            Children = new List<Nested<T>>();
        }
    }

Usage.cs

var nestedList = Hierarchyize.Transform(flatList,
                (c, p) => c.ParentID == p.ID,  //is parent
                (c) => c.ParentID == 0)