Setting expiration for external Javascript

Hello,
I checked my site’s speed using Google Page Speed, it suggest me to set expiration for java script files that are hosted somewhere. I set expiration for all files in my server but I don’t know how I could add expire headers to scripts not on my server.
Here is a screenshot of that→ https://dl.dropbox.com/u/19870375/jsexpiration.png

I would be grateful if someone could help. Thanks
Regards,
Vivek Nath.R
Digital Advices