You can use the Viewer & Aligner to make a copy of your text with all the sentences and/or paragraphs tagged with <S> and <P>.
To do this, simply read in the text file in, choose Edit | Insert Tags, then save it as a text file.
See also: Viewer & Aligner contents
Page url: http://www.lexically.net/downloads/version5/HTML/?aligner_for_numbering_sentence.htm