Its been discussed a few times before and the general consensus is that you are far better off investing in some good quality syrup rather than try to actually flavour the beans.
If you put flavoured beans through a grinder, then it WILL end up tainting the next non-flavoured beans you put through and will continue to do so for a LONG TIME.
Try some syrups. I know Veneziano and Coffee Hit sell them. Im sure a number of other sponsors would have them too.