I was not able to add a suffix to the class name of the module using the module properties in module manager.
I've found a blank space in mod_k2_content:

The file is:
modules/mod_k2_content/tmpl/Default/default.php

row 16:id; ?>" class="k2ItemsBlock ">
You have to delete blank space after k2ItemsBlock if you want to be able to append class suffix.

The new code is:id; ?>" class="k2ItemsBlock">
Bye,
MM

Wow thanks :) I was looking in the components folder...<.< :)