I think that you will be able to input your accumulated distance. The instructions for my Cateye Strada include steps to do so. It says "Input" on the screen -- it's the mode after setting the clock and before choosing km/h and mph.
Cateye instruction manuals are here:
http://cateye.com/en/manuals/66
Your CC-8000 ATC manual:
http://cateye.com/sites/cateye/uploa.../CC-8000_E.pdf
(after looking through that manual)
Hmm... well, I don't know what to say, because that booklet doesn't say anything about adding mileage to that model of computer.