If you have access to a computer with Internet, a USB Cable and your KOBO, you can add a book at any time. Plug in the KOBO and it will appear as a removable drive. Just drag and drop your ePub file.
Now, finding an ePub file may not be that easy, but there are lots out there.
http://www.gutenberg.org has lots in this format, all for free; download, drag, drop and you are done.
If you can get Adobe Digital Editions installed on the PC you are using, you can borrow books from your local library no matter where you are in the world.
Finally, if you just want some backup, if you have some ePub format books that you want to have add later, save them on line in your Google, MSN or whatever account you use. You can then retrieve them and add them as needed.
Remember, you are only "tied" to your home PC if you allow the manufacturer of your reader tie you there