Oh my god, try reading the LSAC website (
http://www.lsac.org/)
Download the information book here
http://www.lsac.org/LSAC.asp?url=lsa...sp#usatitle=on
And the answer to your question is yes, but you only have like 3 days or something. I can't remember. And you can't see your results before you decide - you never see them. Basically, only do this if you just couldn't finish the test at all and you're really sure you did poorly.
If you don't do it in time and just retake the test, most schools will average the results. That's what I hear. I just took it once.