My setup is a 30" Cinema Display (2560x1600, DVI) from around 2007, and a 17-inch, Early 2011 Macbook Pro. I'm wondering if I can add a second external monitor via a USB adaptor or any other solution. Ideally another monitor of the same size and resolution.
Yes, you can add another monitor if you use a USB video adaptor. No, there are no USB video adaptors that support dual link DVI monitors such as the 30" Cinema Display.
Many USB video adaptors will support 1920x1200, some a little bit higher.
However, you will find the USB video to be slow and annoying if you are used to high performance video.
I suggest you consider using two Apple Thunderbolt displays which are a bit smaller and a bit lower resolution at 2560-by-1440, but you can connect two of them, and you'll actually find as good as your cinema display is, technology has improved noticeably since its release, and the thunderbolt displays are absolutely stunning.
You might be able to connect one thunderbolt display, then attach the thunderbolt-DVI adaptor to that display and attach your cinema display.
To that end, you might find some of the information in this question useful:
If you take a look at the Apple support pages, there is stated:
Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
So I don't think you'll be able to hook up two 30" screens (2560x1600) to your MacBook Pro.
But if you set your resolutions to something smaller, you can buy some 3th party hardware to support dual screen modus...
Here is an overview of the available resolutions and the product you can buy (Matrox).