Just about any bike can be fitted with an internal geared hub by replacing the rear wheel. If the has verticle dropouts then you will need some kind of chain tensioner like the Surly Singulator. The most difficult part will be the shifter. I think you can make Shimano style index shifters work with a hub gear but I'm not sure. Otherwise you will have to find room for the included shifter which is usually a grip shifter or occasionally a thumb shifter.
Craig