One of the first things you’ll need to look at is the PR of the current site. If it is not substantial enough and/or spread out enough across the existing domain and you dump a bunch of pages out there, something is going to happen.
It will all be relative to the internal link structure of the site and how those new pages are presented within that structure. And, if the pages are mostly “stubs”, they will suck some life out of the primary pages and at the same time find their way into the Supplemental Index if it still exists.
No, I think you’ll be much better off adding them naturally and at a set pace. Too much of one thing is not good from my perspective. If you have low PR now and dump a bunch of pages, you may not have enough PR to support them and something has to give.
Most of us have found ways to add pages automagically. But, we also realize that adding pages just for the sake of doing it doesn’t always work out as planned. In fact, many times it will work against you if they are not true content pages that would naturally be part of the website.
And then you have to think about any other “signals” you’ve been sending up to this point. If the adding of that many new pages sends another signal in addition to the others you’ve already sent, it may be the one thing that trips something.
There is nothing wrong with adding new pages. There are challenges though when adding “lots” of new pages to an existing website that has an established profile.