As Kati said, this is technically not possible (unless I write a plugin for this, and change a lot of the current moderator API).
I don't really see a reason to do so though.
As said by several people, trial moderators would do everything to look good, but when they have the promotion in their pocket, they will change. This will only cause more issues.
Moderators are picked carefully and are screened by me before any promotion takes place. I also do a few tests with them, without them knowing I am testing their moderator capabilities. I can promise that I take promotions really serious, and I really believe in the moderators we pick.
There will always be complains towards moderators, those turns out false most of the times, luckily.
People will always complain when they receive a infraction, it is the same as hating the police just because you received a fine, which was in your eyes unnecessary, although you know that you were misbehaving.