Add a checkbox to back office products?

[Lpeek]

Does anyone know how to add a new checkbox to the products options? I tried adding to admin/tabs/AdminProducts.php but I see no effect in the back office.

What I want is a new checkbox with an option to add a 'from' before the price only on certain items and this is the best way I can think of. I know I need to do a lot of changes but I thought this first change would be the simplest but it's proving to be the most difficult?

I tried adding this to AdminProducts.php at line 1440 with no luck:

           <input type="checkbox" name="sfrom" id="sfrom" value="1"'.($boolFrom ? ' checked="checked"' : '').' />
'.$this->l('Display price "from"').'

Any ideas or better ways to go about it?