Can I vent a bathroom fan through the eaves?
You should use a separate outlet for the pipe as the existing vents won't be able to cope with the volume of air and you'll probably end up with steam in the loft.

Did you buy a kit or separate components?

If you've got a kit then there should be an outlet grille in that, if not you should be able to get one at the same place you bought the fan and flex pipe.