This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You cannot have one WAR depend on another, so what you will have to do is repackage what you have.
If you have the source code for the original WAR, add it to your WAR project - this in effect merges the two into one.
If you don't have the source, then try this: pakcafe the .class files into a JAR file and add them to the WEB-INF/lib directory or your WAR project. Then take the rest of the original WAR (JSPs, etc) and merge it with your WAR project.