Based on what you’ve said, I would probably create a user library containing your library JARs.
You can create a User Library with Window => Preferences, drill down to Java => Build Path => User Libraries. => Then choose New, and give your library a name => Then add the JARs you want.
Then you can add all the JARs to the classpath of any project you have by right-clicking on the project => Add Libraries => User Libraries, and selecting your library.
Update; to have Eclipse dynamically add .jar files you drop into a folder to your project classpath, you need a plugin called Library Folder ClasspathContainer. Follow these instructions and you’re in business.