Do atelectasis & pneumonia look similar on non contrast chest CT scan?

Can be similar. On non contrast CT scan it is difficult to differentiate in some cases atelectasis from pneumonia. With contrast administration usually atelectasis is more contrasted (enhanced) than most pneumonias.
Sometimes there. May be some overlap in the CT appearance of pneumonia and atelectasis, other times there are signs that help tell the difference. Keep in mind that elements of both could be present simultaneously.