title: ADD a new column into an XTS object
body: <p>Hi: I have an xts object as such: </p>
  
  <pre><code>           AAPL.Open AAPL.High AAPL.Low AAPL.Close AAPL.Volume AAPL.Adjusted
  2013-09-09    505.00    507.92   503.48     506.17    12116200        506.17
  2013-09-10    506.20    507.45   489.50     494.64    26490200        494.64
  2013-09-11    467.01    473.69   464.81     467.71    32031600        467.71
  2013-09-12    468.50    475.40   466.01     472.69    14409400        472.69
  </code></pre>
  
  <p>I try to calculate a rolling mean and attach it to a new column as such </p>
  
  <pre><code>AA["AAPL.Rolling"] &lt;- rollmean(AA[,"AAPL.Adjusted"],12)
  </code></pre>
  
  <p>Although the <code>rollmean(AA[,"AAPL.Adjusted"],12)</code> works on its own; I get an error message when I try to attach to a new column. 
  ** also what makes this hard is that the new rolling mean will not have data in every row since the first 12 should be "NA" Can anyone help?  Thank you much. </p>