DT as you know simply means defaultly trusted members and I think that Theymos is the one that adds them to the DT list by himself. I came across a post this evening while I was surfing the forum. The post way on the meta board and Theymos reply was all over the thread I can't really remember if he was the OP of the thread.
Anyways I don't really know all the criterias to be selected by Theymos in the DT list but I know one of them is that you must be added by an already existing DT member preferably DT1 to be able to get a chance of getting on the list. And yes a newbie could someday be a DT member infact every DT member was once a newbie.