From Roshen on the SOE Live Forums:
It does NOT look like we’ll be able to add rooms to the SOE Live room block. If anything does free up to us before the event, it doesn’t look like it will be at the same prices listed in the SOE Live FAQ.
It looks like some people have good suggestions for other nearby or cheap hotels in Vegas. Before walking the Las Vegas Strip, just be aware that a Vegas ‘block’ isn’t as short as you think it is.
So if you waited til the last possible moment to book SOE Live and pay at the door, you’ll want to shop around other hotel options.