I called my cc company before I booked online. I don't know if that helped but it all went through fine. Just have google translate open in another window incase anything isn't obvious.
I have to say I also wouldn't recommend the meal plans unless you particularly want to get your meals paid for in advance to help with budgeting. Unless you stick to the restaurants in your chosen category there's no saving to be had plus you're paying for three big meals each day plus lunches. You might not fancy a full buffet or three course meal each day when you get there. Also, I've found the counter service offerings to be pretty generous so you might be happy to share meals or downsize. Next time I go I think I'll be ordering a children's meal for me as they're plenty big enough.
But to answer your actual question, yes you should be able to call Disney and quote your booking reference if you want to add anything on to your holiday.
Good luck with the booking!