The only sure fired way to get your doctor to diagnose you as needing diapers, is to have regular, uncontrolled accidents, wet or messy. There is a condition called diverticulosis or diverticulitis and it causes irritable bowel syndrome, or uncontrollable poops, in your pants or diapers. It's very hard to diagnose and a lot of physician's, if people have nervous bowels, get diagnosed with this.
It is not very controllable, and can cause explosive bowel movements.
The other way, is to start drinking liquids that are considered diuretics. A diuretic is a fluid or liquid that causes the bladder to want to empty more frequently.
All types of teas are diuretic's. You can also get products like Diurex, which is a water weight gain pill for women at times of their period, that put on water weight. It causes the bladder to fill and want to empty with fluids.
If you can get your hands on a prescription diuretic, like Lasix or other prescription medications, you'll need to have a lot of fluid on your system. A lot of times, diabetic's will have swelling of the legs and lower extremities and the doctor will put them on a diuretic to push that water out of their system.
The only thing I know, is you're going to have to be dedicated to this, once you start and not let anything get it your way.
Be careful with previous suggestions of mentioning bladder or kidney problems to your doctor, they can mean a long battery of tests to check for potential, sometimes life threatening problems.
Stress Incontinence is the release of the bladder fluids under stress!
Urge Incontinence is the release of fluids when pressure or laughing occurs. A lot of women suffer from this, especially after child birth.
You can also train your bladder muscles to relax. If you try and keep your bladder in a relaxed state, it will just naturally release, when it feels full.
Practice by not pulsing, that final once of twice when you finish urinating. Remember the relaxed state of your penis and bladder. Don't do the pull and jerk at the end, just stay relaxed as long as you can. When you feel you have to pee, just relax and let it go in your pants. Try and stay relaxed as long as you can. Eventually, try wetting while sitting, laying and standing.
Eventually, you're body will relax and wet natually, anytime, anywhere you want to.
It is easier to fake messy accidents, than wet ones. Using stool softners or a laxative will give you the desired results and probably get you in diapers faster, since they are messier and smellier than a wet accident and more plausable.