You can sometimes train yourself to wet a diaper in your sleep and it's just a matter of mind over matter.
You can begin by starting to wet your diaper while you drift off to sleep or start wetting your diaper while laying in bed awake.
Your mind has been trained to not allow you to wet in your sleep or in your bed so you just have to train your brain that it's okay to wet while lying in bed.
To wet your diaper while asleep make sure that you have a full bladder so drink plenty of water before bed so you have a better chance at wetting the diaper while asleep.
It can take a few months or even a year to fully train yourself to wet the diaper at night without knowing about it so keep trying.
You might also try some hypnosis tapes to help you train yourself to wet the diaper as I hear it works for some.