Yes you can. Your membership is valid on both but you must contact Celebrity Captains Club to get your 'New Number' either by phone or eMail.
No you can't!. The two schemes are seperate but at a certain level they have a reciprocal agreement. You have to be Platinum member of C&A (5 points) and then you can call Captains Club and they will give you a new number. You will join at the equivalent level, called Select, where you will only have one point after your first cruise. You collect and accrue points in each system seperately. If you are just a gold member with C&A, then sign up for Captains club when you are on your cruise. Think Tesco and Sainsburys Club cards - different companies so different schemes! And the benefits on Celebrity are not nearly as good as those on RCI C&A.
Sorry I wasn't aware you had to be a minimum of Platinum, as we were all above that level when the scheme came in. I do agree about the benefits though, I don't think we used one coupon last year. Priority Boarding was about the only useful part for us.