I want to edit the li-elements of a menu block

So I have my Primary links and it has a lvl 2 that I chose to show in a sidebar (region called "sidecontent") as a block for each respective site. The menu shows up fine, but I would like to change the class names for the li-elements and also add an extra li-element as the first leaf with some custom text.

How would I proceed to achieve this?

example:

<ul class="someclass">
<li class="customclass1">Custom Text</li>
<li class="middle">menu link 1 from menu block</li>
<li class="last">menu link 2 from menu block</li>
</ul>

edit: nvm I changed the design & layout instead. Seemed too much hassle.