Connect two 30" ACD on a 17" MacBook Pro (early 2011)
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:

How to connect two non-Thunderbolt displays to the Macbook Pro 2011 via Thunderbolt?