LEW is all SRD and house rules. Open source content can be voted in by the concurrence of 3 judges, start a proposal thread. Non-open source content cannot be used for copyright reasons, but independently-generated material that is similar to Non-open source content yet sufficiently different enough to avoid copyright infringement may be voted in as a house rule (again, with a proposal thread and 3 judge's OK).
As an example, if you wanted a Red Wizard of Thay, that's not in the SRD. It is in the DMG, but the DMG is not open source content. So, short answer, you couldn't have a "Red Wizard of Thay" in EnWorld.
Now, you could have a "Blue Wizard of Fallon" that was somewhat similar to a Red Wizard of Thay. "Somewhat" similar means that you couldn't just copy the Red Wizard of Thay description out of the DMG and change "red" to "blue" and "Thay" to "Fallon" everywhere in the description, you actually have to write your own PrC that may borrow some of the concepts of the DMG RWoT.
You then put your "Blue Wizard of Fallon" into a proposal thread, people comment on it, and eventually (days to months) the judges get around to approving or disproving it as part of EnWorld.