There's some deep magic that might allow you to copy the raw data of the filesystem and then expand it, but I can't see what possible benefit it would have to offset its likely flakiness.

No matter what you do, you're going to have to copy massive amounts of data, so it's going to take a long time, no matter what manner you choose. I'd use tar, since it's a Linux system. Make sure to use the right options to keep owners and permissions and whatnot. You might want to grab a copy of star; it's likely to be faster than gtar. If it was another Unix, I'd definitely use dump and restore, but those are a little flaky on ext2.

This, of course, assumes that I understand your question properly and that this is going to be a brand new filesystem, and not that you're trying to add more drives to an already existing RAID solution. That's a different story.