Welcome to the forums, newcomer!
You can do what you want to the case as a mod, though you're going to need a jig or a template to get the holes done right and to allow for proper airflow (for the air to pass through radiator and out the top) otherwise you're essentially impeding the air from escaping.
Judging how you're asking would indicate that you don't know what to do or how to do it. If that's the scenario, don't bother ruining the case, sell it and pick up a case from Fractal's Torrent series or a case that has unhindered top mounting options.
My 2 cents on the matter.