Creating a NDT column by concatenating two fields

[sundarn]

Hello, 

I want to create a column in an NDT that concatenates two fields. 

view: ndt_sessions {
  derived_table: {
   explore_source: sessions {
      column: website_name {}
      column: partition_date {}
      column: id {}
   }
  }
}

I want to derive the column id{} from two fields fullvisitorid and visitid that are not in the table yet. 

Can I create the column id as a concatenation of the fields fullvisitorid and visitid without adding these two fields as columns in the table? Or do I have to bring those two into the table and then derive the id column?