Upgrade 2.9 to 3 beta with multiblogs?

I have a test blog that I’ve maintained for several years, upgrading it with each new release, to test on before rolling any code live to my actual blogs. I wanted to develop a theme that was ready for 3, so I upgraded my test blog to the latest beta, but I didn’t see any options to upgrade it to a multiblog. Is that possible? After it’s been upgraded, can it be moved to a multiblog? Or am I just stuck?

Thanks in advance!