You can't override the built-in ones. The best you can do is deactivate them and replace them with your own subclasses of UIActivity. Functionally, this would have the same effect as overriding them.
Visually however this means that you have to provide your own icon images for these services. Apple's code will then take these images, throw away the colour data and use the alpha map to make a greyscale-tinted version set against Apple's usual black-dotted background. You can't provide colourful images for your own custom activities. Presumably this is to stop people from copying the 'official' service icons and subverting them for their own purposes, i.e. pretending to post something to Facebook while actually doing something quite different.