with any pure CSS hack it’s not possible.
Use IE8.js http://code.google.com/p/ie7-js/
It has support for this. http://ie7-js.googlecode.com/svn/test/index.html
test page also there
after – http://ie7-js.googlecode.com/svn/test/after.html
before – http://ie7-js.googlecode.com/svn/test/before.html
Edit after 1st comment
You can just keep this js for IE6 and 7. other browser will not read it.
<!--[if lt IE 8]>
<script src="http://ie7-js.googlecode.com/svn/version/2.1(beta4)/IE8.js"></script>
<![endif]-->
And if you are already using jQuery in your project than you can use this plugin
jQuery Pseudo Plugin
http://jquery.lukelutman.com/plugins/pseudo/