you would need break open that adapter listed HERE
The adapter listed here is for use with 2 stereo headsets, you would need to modify it so that the stereo leads on one side of the female 3.5mm plug are soldered together and that they are on there own contact on the male 2.5mm plug.
Then you would need to make sure the other female 3.5mm is setup the same way so that the mike is connected to the ground and 1 contact on the male 2.5mm plug.
Email me if you need me to clarify, i'll do a rough sketch when i get a chance.
PS one issue you may run into is if the headset will be loud enough, but then again that is what testing is for, i would imagine since its the kind that seals to your head you should be good.