Footnotes will use the column layout of the page style or section from which they were introduced, so there is no easy way to do what you need.
As alternative you can set your page style to have columns but insert the text on a full page width frame, but of course this gives problems with text flow between pages... Frames can be linked, so when one frame is full the text flows to the next one, but if you have hundreds of pages this means hundreds of frames so it is not a practical alternative.
So no, it is not possible, sorry.
There are two types of people: the ones that believe that there are two types of people and the ones that not.
openSUSE 12.2 with KDE SC 4.8.5 / AOO 3.4.1