I want to install a motion sensor for the stairwell
Need some more info:
What are you trying to do? I mean, if you want the 3-ways to control on and off as they do now, and have the motions apply lights regardless of the 3-ways being on or off.
Can you add wiring, or are you intending to try and do this using only existing boxes and wiring?
There are probably several configurations of function possible.
Also, I know the code requires ability to turn lights on from any access/entry point in a stairwell; as far as I know it does not require ability to keep it off...
(so combining 3-way control and motion control is ok AFAIK)
[I do not know this for sure].
What I assume that you want to do, would require at least one additional conductor, and probably 2, possibly 3, depending on existing wire routing, and if 2 or 3 wire motion sensors. (and require 2 motion detectors, standard kind - single pole).
Frankly, I did not understand your sensor that can use one 3-way; a 3-way would not function alone.  I would need details on that to understand what it is, or does.
{there are code implications, depending on what that actually does, or if it has a manual 3-way function incorporated).
Mr K L
OOPS, I'm new here.  I should have posted this as a 'comment', as it is not an 'answer' yet...
I will try to move it.
-- Huh, I can't move it until I have '50' reputation; but it lets me post incomplete 'answers'; oh well...