On double checking this beorg already sets the start and end time for a task in the calendar. I misread what you were asking, what you want is to be able to set an end time in beorg. I do need to take a look at the date selector as currently misses not only the end time but also warning periods (which also need to be better supported in the agenda) and for habits the minimum repeat interval.
That's a good idea. Even though beorg doesn't currently support editing a time range it does understand them so it would be a straight forward change to the calendar entry. I'll take a look into this relatively soon as a nice simple enhancement.