I think it is entirely up to the political coalition to determine where those excess votes go.

Parties can have different methods of selecting who is on their potential MK list, and these range from internal party voting to public voting at party primaries.

These lists follow a closed system where parties decide (or decide how they want to decide) who is on their given list.

Because the list, and how it's made is determined by parties, I think additional seats obtained but not planned for would be up to the winning parties to decide. The parties in the winning coalition can determine who to add to the list in internal deliberations or public primaries, I think.