nrow = size(YourTable,1);
YourTable.NewColumnName = zeros(nrow, 1); %0
YourTable.OtherNewColumn = char(zeros(nrow,0)); %empty string, see note
YourTable.ThirdColumn = repmat({''}, nrow, 1); %cell array of empty string
Note: if you use the char(zeros(nrow,0)) then although you have initialized to empty strings, all of the columns then become constrained to be empty strings and you cannot expand those except by replacing the entire column. You probably want the cell array version rather than the empty-string version
