Yes, you can add the Login with Facebook button to that module. It's pretty easy actually, but I can see that we don't have documentation for that yet.
To do so, please copy /modules/mod_k2_user/tmpl/login.php to /templates/<YOUR_TEMPLATE>/html/mod_k2_user/login.php . Create any directories that don't already exist in that path. That creates a 'template override' which makes sure your changes don't get erased when you upgrade K2.
Then, edit that file. Around line 37, you'll see:
<input type="submit" name="Submit" class="button" value="<?php echo JText::_('K2_LOGIN') ?>" />
Add a line below it, so it looks like:
<input type="submit" name="Submit" class="button" value="<?php echo JText::_('K2_LOGIN') ?>" />
<p>or {JFBCLogin}</p>
Obviously, you can change 'or' or add other text if you'd like. Additionally, you could use your own custom image if you'd like as well, instead of the standard 'blue button'. To do so, replace the {JFBCLogin} text like so:
<p>or <a href="javascript:void(0)" onclick="jfbc.login.login_custom();"><img src="/link-to-your-custom-image.jpg" /></a> </p>
Hope that helps, but if you run into issues or have any other questions, just let us know!
Thanks,
Alex