手動でJoinってこういうこと？

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
  class Program
  {
    static void Main(string[] args)
    {
var items = new[] {
  new { Category = "A", Year = 2012, Param = new[]{ 1, 2, 3}},
  new { Category = "A", Year = 2013, Param = new[]{ 4, 5, 6}},
  new { Category = "B", Year = 2012, Param = new[]{ 7, 8, 9}},
  new { Category = "C", Year = 2013, Param = new[]{ 3, 4, 5}},
};
var rank = new[] {
  new { Category = "A", Lv = 4 },
  new { Category = "B", Lv = 3 },
  new { Category = "C", Lv = 2 },
  new { Category = "D", Lv = 1 },
};

var lookup = items.ToLookup(x => x.Category);

var q = rank.Select(x => new { Categoly = x.Category, Lv = x.Lv, Values = lookup[x.Category].Select(y=>y) });

q.Dump();
    }
  }
}