linq2sql: how to modify not-mapped entity member in query?

Question

How to modidy entity in query? I have code like this:

var res = from p in dc.Folders
          group p by p.FolderId into gr
          let DDate = gr.Where(a =>a.status ==5).Max(g=>g.MDate)
          from c in gr
          select c;

Folder class has extra member not mapped to db called DDDate, just for example. How I can set this member in query? If I drop let keyword and calculate&assign DDate in separate forearch cycle, it will be too slow.