If it is possible to retrieve the file name of the previous and following xhtml file within the file list of the EPUB than it shoud be doable. In this case in the following html snippet only in the "href="XXX.xhtml" the "XXX.xhtml" needs to be replaced once by the file name of the previous file and once by the file name of the following file. Inserting this code snippet in every file before the </body> tag should do the job. Shouldn't it?
Code:
<div class="wh_prev_next"><a class="wh_prev_next_link" href="XXX.xhtml"><span class="icon-arrow-left"> </span><span style="vertical-align:top; font-size:large;">Previous</span></a><a class="wh_prev_next_link" href="XXX.xhtml"><span style="vertical-align:top; font-size:large; padding-right: 1em;">Next</span><span class="icon-arrow-right"> </span></a></div>