The classes output by the function already give enough control over CSS.
If you want to change how the functions operates then you could write your own function (some themes do this) or as i said, use echo=0 and do some PHP magic on the output.
For clarification: Example of code use
HTML & PHP
<ul class="mypages">
<?php wp_list_pages() ?>
<ul>
CSS
.mypages {}
.mypages li {}
.mypages li ul {}
.mypages li li {}
.mypages li.current_item {}
.mypages li.page-item {}
If you want to add more classes, then the issue is not with the code, but with your understanding of how to reference the existing classes via CSS (not intended as an insult, so excuse the blatantness).
If you need specific functionality, then please clarify exactly what it is you'd like it to do.