Just add them like normal numbers. You may want to use the [h]:mm:ss number format in order to see sums of more than 24 hours. The regular time formats don't show days, but Excel converts every set of 24 hours into one day.

Let me explain how to time can be added in excel. Suppose there are two cells A1 and A2 and you want to add time and display in A3 cell. Then you have to use this format A3 =SUM(A1:A2).

The best way is to convert it to seconds before adding it up, then split it to hours, minutes and seconds after.