lol If they won't cut the hole for you they should have a hole saw you can rent from them. The doors are not thick they are just two panels with wood support in the middle and some glue.
If it is a normal door they are about $30 buy one at home depot and already have the plans on where you want to holes and how big then they will cut them out for you. that way you dont mess up the matching door that goes to the house.
Well you and your plants lol Walk in closets are one of the hardest things to exhaust because you don't want to cut any holes in the wall. Have you thought about using the door to mount a inline fan for exhaust? You can cut holes in it then just replace it for most likely less than $100 when you are done.
When you want to respond to this move your mouse to the top right and click view conversation. to pm me you click the link under the picture of steve-o. lol hope this helps