title: Add indexed column to DataFrame with pandas
body: <p>I'm a beginning pandas user, and after studying the documentation I still can't find a straightforward way to do the following. </p>
  
  <p>I have a DataFrame with a pandas.DateRange index, and I want to add a column with values for part of the same DateRange.  </p>
  
  <p>Suppose I have </p>
  
  <pre><code>df
  
                              A         B
  2010-01-01 00:00:00  0.340717  0.702432
  2010-01-01 01:00:00  0.649970  0.411799
  2010-01-01 02:00:00  0.932367  0.108047
  2010-01-01 03:00:00  0.051942  0.526318
  2010-01-01 04:00:00  0.518301  0.057809
  2010-01-01 05:00:00  0.779988  0.756221
  2010-01-01 06:00:00  0.597444  0.312495
  </code></pre>
  
  <p>and </p>
  
  <pre><code>df2
  
                       C
  2010-01-01 03:00:00  5
  2010-01-01 04:00:00  5
  2010-01-01 05:00:00  5
  </code></pre>
  
  <p>How can I obtain something like this:</p>
  
  <pre><code>                            A         B    C
  2010-01-01 00:00:00  0.340717  0.702432    nan
  2010-01-01 01:00:00  0.649970  0.411799    nan
  2010-01-01 02:00:00  0.932367  0.108047    nan
  2010-01-01 03:00:00  0.051942  0.526318    5
  2010-01-01 04:00:00  0.518301  0.057809    5
  2010-01-01 05:00:00  0.779988  0.756221    5
  2010-01-01 06:00:00  0.597444  0.312495    nan
  </code></pre>