You dont have to alter the classpath to include third party jar files.
Placing them in the folder you said should do the trick, and has done for me when using the mysql connector jar linbary. Is the problem maybe that you arent referrencing the correct library when you try to use it?
You'll need to use the full qualified classpath in your J2EE import statements.
if i dropped some third party jar say p6spy.jar or quartz.jar in default/lib directory;it didnt recognize that jar;
Have you restarted the JBoss instance?
Ive been using the mysql jar in the same way, and dropping it into the lib directory did the trick.
Have a look through the log files for anything to do with the jar files - try searching for p6spy or quartz - if it is in the correct place, there should at least be a log entry with more info.