title: How to transform some columns only with SimpleImputer or equivalent
body: <p>I am taking my first steps with scikit library and found myself in need of backfilling <strong>only</strong> some columns in my data frame.</p>
  
  <p>I have read carefully the <a href="https://scikit-learn.org/stable/modules/generated/sklearn.impute.SimpleImputer.html" rel="noreferrer">documentation</a> but I still cannot figure out how to achieve this.</p>
  
  <p>To make this more specific, let's say I have:</p>
  
  <pre><code>A = [[7,2,3],[4,np.nan,6],[10,5,np.nan]]
  </code></pre>
  
  <p>And that I would like to fill in the second column with the mean but <strong>not</strong> the third.  How can I do this with SimpleImputer (or another helper class)?</p>
  
  <p>An evolution from this, and the natural follow up questions is: how can I fill the second column with the mean and the last column with a constant (only for cells that had no values to begin with, obviously)?</p>