We cannot add additional buttons because that involves not only circuit changes but also changes to the enclosure. The current manually start program feature is pretty straightforward:
1. long press B3 to enter ‘start a program’ option.
2. click B3 to select a test program or any of the preset programs.
3. long press B3 to start the selected program.
You can create a program with the custom timers you mentioned, and just ask the landscaper to start that program each time.