Can I add xmpp python library to my google app engine server
I don't know but your two biggest limitations will be the inability to use native libraries, and the fact that GAE only supports HTTP requests, with no access to the underlying sockets.
If your library uses native code, or relies on using sockets for communications, you won't be able to use it.  If it's pure python and can work with an HTTP based transport, you should be ok.
I'm trying to send messages between xmmp clients using gogle app engine as a server, for that reason I prefer to use xmpp library for python (xmpppy) instead the xmpp library of the google app engine API. Can I add the xmpp python library to my server? I mean can I use this library instead the xmpp library of google app engine?