how do you sync apps from another itunes without erasing all your apps

Ok so here is my situation, my usb is down so I am downloaded an application on another computer and uploaded it to Itunes. Now I got it loaded up and trying to sync it to my ipod but it wants to erase all my other apps and just put the one on. So how do I get just the one app on without erasing it, would appreciate the help, thanks in advance!