Add Existing Series works, UI shows TVDB Name and Path so you can check before adding to DB.

NzbDrone.Core/Model/SeriesMappingModel.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace NzbDrone.Core.Model
+{
+    public class SeriesMappingModel
+    {
+        public string Path { get; set; }
+        public int TvDbId { get; set; }
+    }
+}