I have read and avoided threads of individuals who have reported that adding new content to an existing domain has triggered the domain to be "sandboxed." This time, let me point you to an example of a new thread that sprung up on the topic at WebmasterWorld named Adding Large Amount of Content. The thread creator asks if his site will be sandboxed if he adds more "useful" content to it, like 15,000 pages in one day.
As you read the thread, you here some reporting that this has shot them into the sandbox. In contrast, you have some reporting that this technique has shot them out of the sandbox. And then you have those that say, they simply rank well for the new pages they add within a few weeks. Some place a distinction between adding a small number of pages at a time (about 10) and some say it doesn't make a difference either way.
These types of threads make for interesting reading but rarely have any bottoming out.