You can't have both numbers and characters in the same column, which is what you were trying to do. This works:
colHeaderExcel = {'Time_hour','Speed_ms_per_sec', ...
'LevelTime_hour','Level', ...
'Entries','TotalItem','BypassItem'};
reportMatrix = rand(9, 7);
t1 = array2table(reportMatrix,'VariableNames',colHeaderExcel);
t2(1,1).Time_hour = 0 ; '\nStatistics:\n\nAverage Write: xx\nAverate Read: yy\n';
t2(1,1).Speed_ms_per_sec = 0;
t2(1,1).LevelTime_hour = 0;
t2(1,1).Level = 0;
t2(1,1).Entries = 0;
t2(1,1).TotalItem = 0;
t2(1,1).BypassItem = 0;
t2 = struct2table(t2)
t2.Properties.VariableNames = t1.Properties.VariableNames;
t3 = [t1; t2]
t4 = cat(1,t1,t2) % Alternate operation
