Although most people provide the source code for their libraries it is not always made available in a format that can easily be imported into Eclipse.
In this case I would recommend creating a new Eclipse project using the libray template from
http://dev.processing.org/libraries/template.html and then copy the library source files into this project.
To create your own library then download the Eclipse template from the link above and then create your code.
To build your library in Eclipse then from the menus select
Window | Show View | Ant this will open a pane on the right hand side. Drag the build.xml file from the Project view under resources to the Ant pane , click on the + to expand the view and double click on the
Build target. This will ultimatetly create the zip file that can distribute to others.
Before doing this you will want to edit the build.xml file for your specific library settings.
Also the the Eclipse project settings must be Java 1.5 compliant (not 1.6) if you want to be able to make it available for others to use.
Also
JRE System Library in the Eclipse project must point to a jdk not a jre for the build to work. Otherwise it will hang up when it tries to build the javadocs.
Hope this helps.