On the Project Properties -
Java Build Path - Libraries tab, eclipse lets you add stuff to your classpath that is external to any eclipse projects three ways that I see off hand. Each is a done using a button on that libraries tab. You don't give a lot of details so not quite sure what/why you are trying to accomplish.
1. Add External jars ... self-explanatory, browse and add external jars to classpath
2. Add Library ... this one is a little more work
3. Add Variable ...
If you're trying to get a folder/directory on the classpath so some properties or other files can be found then add variable should work.
When you use the add variable button (I'm using v. 3.1, I think this works a little differently in earlier versions) there is a "Configure Variables" option. When you do that it lets you add variables. Give one a name like you're creating an environment variable and browse to the directory/folder. After you've created it, you'll be able to add it to your classpath.