In all honesty, we need a bit more information about the setup you hope to achieve at the end of this in order to offer specific advice - plus it also hangs on whether or not the third party code you need to use is already hosted in a subversion repository or not.
There's a good setup described in the red-bean book here: http://svnbook.red-bean.com/en/1.8/svn.advanced.vendorbr.html
If you need more branches, or the code you need isn't available by subversion then you'll need to create the branch in your working copy, add the code yourself and commit it back. Once that's done though it should work like any other branch, and the merging should be fine as described in the article.
If the article doesn't help please give us a bit more information about what you need and what's missing and we'll do what we can.