As I explained in my other comment, that doesn't have to be a problem. Just let everyone vote with a traditional "single choice" ballot. But you also allow the voter to optionally enhance/refine their preference with a score ballot. If a voter chooses not fill out a score ballot, their "single choice" ballot will be automatically converted into the corresponding score ballot. Their vote won't count any less. It will just be a (likely) less complete representation of their actual preferences.