Yes you probably can if you are clever, but if they catch it, you will be terminated and all the hard work you put in to building your sites will be gone forever. They offer such a cheap upgrade it's silly not to take it, it's like 4 or 5 dollars a month, then you can make up to 10 sites.
Or better yet, instead of taking advantage of a great company that gives all of us so much for free. Make your 2 sites at weebly, then make 2 more over at WebStarts, it's very similar to weebly, you can do 2 free, and you won't be risking losing it all.
Best of luck,