Personally, I'm planning on installing v2.0-final, uploading whatever I need to, and then installing v3.0-alpha5.

I don't know for sure, but I seem to recall that this won't work. 2.0 and 3.0 use different database types, yet 3.0a5 won't rebuild databases. So if you're going to go "down" to add content, it's gotta be down to 3.0a3 rather than 3.0a5.

There is also the manual database rebuild that others have described, so you could look up how to do that.