sqlite: how to add total time hh:mm:ss where column datatype is DATETIME?

SQLite has no native DATETIME data type; the values are stored as strings.

You have to use a built-in date function to convert from these strings to a number of seconds before you can add them:

SELECT sum(strftime('%s', total_expend_time) -
           strftime('%s', '00:00:00'       ))
FROM timetable

If you want to have this formatted as a time, you have to convert it back afterwards:

SELECT time(sum(...), 'unixepoch') FROM timetable