I googled a bit and found that it can be done by services--->servers--->add new, but when I try to add new Tomcat server this way, it doesn\'t appear in the list.
I had same issue. No need to re install.
In Netbeans 6.0 , Find RunTime -> Servers - > Add server -> select Tomcat install 'root' directory
In Netbeans 7.x -> Tools -> Servers-> Add server -> select Tomcat install 'root' directory
Here is in Netbeans Wiki.
http://wiki.netbeans.org/AddExternalTomcat