You may do this:
- login to your .com blog
- look for tools ( export) where you will find two options. Option one is where you have to do the work yourself. If you are not pretty sure, you may encounter problems in exporting from there and then reimporting to your current blog, changing URLs, etc. The second option is that for a payment ($129 or so), the WordPress engineers will transfer your blog to your current site.
If you are going to do the work by yourself, I suggest to do the work using a test site or a local WordPress installation (on your computer) and once you find the blog is correctly imported, you may then re-import to your present site. The work may seem a little complicated if you have not done it before. But by trying and practicing, you can perfect the work.