Hi,
I think that it is a lot easier to increase the chamber's RH than reducing it.
A tub of salt is often recommended for reducing RH; the salt absorbs the moisture and you then remove the salt to dry out (replacing it with dry salt, so you have a cycle of salt removing moisture).
I tried this and I found it to have limited effectiveness. In other words, it didn't really work for me.
To add humidity, I suggest a couple of large, wet, sponges (or towels) in a bowl at the bottom of the fridge. Anything that will gradually dry out will have the effect of releasing moisture into the chamber's interior.
Another good idea is to use Google to do a site-specific search on this site for other people's methods of increasing humidity.
Best wishes
Steven