You cannot sum "time" fields only "duration" fields, so some numerical type.
Hint: You could look for/create some data-element/domain, internally a NUMC type in seconds, with a conversion-exit to display/read it in a "time" format. You could look at FM CONVERT_TIME_INPUT to create your own ZDURATION.conversion exit) - You could also add a "days" part in the format to prevent overflows.
Regards,
Raymond