Is it possible to change pandas column data type within a sklearn pipeline?
You sure can.

One solution off the bat is to extend your estimator that 
 takes object type variables. So what does that mean.

Library that you said are all estimators in the sklearn form fit, predict methodology. So all you have to do is something as follows:

&gt; class modifiedTraf(oldTraf):
&gt;        def __init__(self):
&gt;           super(modifiedTraf, self).__init__():
&gt; 
&gt; bla bla   
&gt; 
&gt;       def fit(self, X):
&gt;           X[col] = X[col].astype(object)
&gt;           super(modifiedTraf,self).fit(X)
&gt;           return self
&gt;       def transform(self, X):
&gt;           X[col] = X[col].astype(object)
&gt;           X=super(modifiedTraf,self).transform(X)
&gt;           return X


Learn more about overriding sklearn classes and super argument for example here