directly MAY NOT be a good idea. It can cause warpage as Roham pointed out.

However...if your humidor is SOLID cedar then it probably won't hurt it for the initial conditioning. It's easy to see if your humi is made of a single layer of wood. Most inexpensive ones are.

If your humidor is a more costly one it may be a box within a box, e.g. a hardwood exterior lined with spanish cedar. If this is the case, you are asking for trouble if you wet the interior as previously stated. It will cause the interior lining to warp.

Also, regardless of the type of humidor, wetting ANY kind of wood causes the grain to become raised creating a somewhat rough surface.

If you have the patience to wait a week, do it as Roham suggested. Use a small glass filled with water or use a very damp, CLEAN sponge placed on a small tray or dish inside the humi ( a bit better ). Leave it closed for a week or so and the wood will take up the moisture slowly and naturally. You can then begin using the humidor to store cigars.

Always use distilled water.