But why would you want to? It makes more sense to convert the capture files
in the other direction.
Not neccessarily. He might have to send some captures to, say, a tech
support engineer. Often they want something Sniffer can read (or at
least they use something that can read Sniffer format files).
Been there, done that.
To answer the original question: look at 'Save as' in Ethereal or the
'-F'-option of editcap.