title: Adding sub columns to every single existing columns in pandas
body: <p>I have a data frame with over 2000 columns with complicated names. </p>
  
  <p>Each column has a price value, with an index of time and I want each column to have additional sub column of 'r'.</p>
  
  <p>For ex. </p>
  
  <pre><code>col_1  col_2    col_3   col_4  ...........   col_n
  </code></pre>
  
  <p>to be changed into </p>
  
  <pre><code>col_1  col_2    col_3   col_4  ...........   col_n
  
  I price I r I .................................. I price I r I
  </code></pre>
  
  <p>I have tried multi index method, but I am having trouble. </p>
  
  <p>All the columns have a price, and I just want to use existing values for  the new price column, and add extra sub column called <code>k</code>.  </p>