You can do that by pressing the left and right button on the back. Yes, the dial is in full stop, but with the left and right arrows, you can change in 1/3 steps in either direction.
This is in the manual btw.
Great pick-up mouren. That's another work-around happily. But, that's all it appears to be, a happy coincidence.
I checked the manual and under Manual Exposure p.27 it makes no mention of being able to adjust the shutter speed in 1/3 increments, with the use of the selector switch or in any other way. Good pickup.
Although, it is mentioned under Program AE (P) p.24 and Long Time Exposure p.28
For Program AE (P) I quote:
"Program Shift : If desired, you can press the selector left or right to other combinations of shutter speed and aperture
without altering exposure (program shift)."
It goes on to say that this is not available if "an auto option is selected for ISO in the shooting menu". Well, program shift isn't but changing the exposure is. The selector switch will change either the ISO (if Auto ISO is selected) or shutter speed as you picked up.
This appears to be just another undocumented operation of a camera whose firmware appears written by non photographers whose only previous camera experience was with their phone.
For Long Time Exposure (T/B) it again does not appear relevant, being available to select speeds between 2 and 30 seconds.
Like the operation of the AE-L/AF-L button I find many aspects of the XP1 incomplete, illogical, complicated and often totally random
(undocumented).
As a result I am waiting on this M adapter and new firmware in the hope things improve. As it stands and without Adobe raw support I will use my other cameras and leave the XP1 on the desk.