I've created dummy data that from what you're saying is similar in structure to what you use.
The result of the code ends up like this:
You want to add a single row to the top of the field value array?
If so it can be done in the loop using something like this:
Dates = datetime(Data(1,:),'convertfrom','excel');
Data(1,:) = datenum(Dates);
Data = [Data(1, :); zeros(1, size(Data, 2)); Data(2:end, :)];
GPS = setfield(GPS, Player, Data);