Yes, that allowed me to add values of two columns, but I still don't get the expected result.
INSERT INTO Processes(Name, StartTime, DurationTime, EndTime, IsFinished)
SELECT 'AAA', st, dt, st + dt, 0
FROM (SELECT '2018-01-22 09:00:00' as st, '0000-00-00 00:05:00' as dt) Processes;
The expected result for EndTime column is: '2018-01-22 09:05:00'
The actual result is: '2018'
Of course I declared EndTime as TIMESTAMP