I’m not sure I fully understand your question, but here goes an answer anyway.
I think you are asking whether the MT folks can install MT onto your website without the rest of the content being there, yet. If so, the answer is yes.
When you ftp into your account, you will find yourself in your home directory. If you have already added the new domain via the DreamHost control panel, you will see it if you list the contents of your home directory. The MT folks will need to install MT into a directory inside the domain directory.
If you want to do a quick test, you can always create a sub-domain off dreamhost.com (e.g., mycrazysweetinsanesubdomain.dreamhost.com). Since the DNS updates only have to occur on DreamHost’s servers, that sub-domain will become publicly visible much quicker than a full domain (which may take a couple days to propagate to external DNS servers). If you look in your home directory, you will then see a sub-directory called mycrazysweetinsanesubdomain.dreamhost.com. Of course, if you are paying someone to install MT, you probably want to wait until you have the real domain ready to go. While you could always move the MT directory and update the config file settings, if you aren’t comfortable installing MT by yourself, you probably wouldn’t be comfortable doing that.
And if you change your mind about paying someone to install MT, you should use my install notes. Though the notes are for MT 2.65, they also work fine for MT 2.661, assuming you change the file name, e.g., MT-2.65-full-lib.tar.gz becomes MT-2.661-full-lib.tar.gz. I recently used these notes to install MT 2.661 in less than 10 minutes, start to finish.