Especially since I would have to get one for each room.I'm also considering one of those portable air conditioners, but they still need to vent outside. 

Closet door that would fall out of the tracks when moved.There are many more issues in addition to ones mentioned above & they would not provide me any proof or evidence even after emails & letters.

Are there any rooms that I could convert to a bedroom by adding in a closet and/or an egress window? 

So things like overhead lighting, plenty of outlets, and ample closet space in my mind make a big difference as well.Depending on the rental pool and where it is in the country, central air might make a difference as well.

A small electrical issue in one closet. 

Generally speaking your bedroom needs to have at least 70 square feet of floor space (if more than one occupant in the bedroom need a minimum 50 sq. ft. per occupant), ceiling needs to be at least 7 feet tall, can't measure less than 7 feet in any horizontal direction, needs a heat source, a light source, an emergency/rescue opening, its own door (can't access it through another bedroom), and many people will say it needs a closet (the IRC doesn't have this requirement, but many appraisers use it as a bedroom requirement).If you want to read through the code yourself, I've copied it below:IRC- SECTION R202 DEFINITIONSHABITABLE SPACE.

Tenant is moving in May 1st and asked if the walk-in closet door can be made to open to the outside.

You might be able to sue for damages as well for the costs above and beyond normal wear and tear.My advice - step into a closet and scream profanity at full volume. 

Does anyone have a favourite closet system to install when your bedrooms don't come with one?

Now, check your water flow (presure) at the closet hosebibb which is nearest to your water meter.