Are both of the accounts authorized on the same PC?
For example, if someone else in your household has iTunes on the PC and you want those songs too, you just have to authorize your iTunes account on that PC and you can sync to that library.
Otherwise, you can locate where the songs are on the PC ( right click a song in the playlist and select show in Windows Explorer) and copy those songs to CD, import them to your PC, and unless they are DRM protected, you should be good to go.
If they are DRM protected, then the "owner" of those songs has to be authorized in iTunes on your PC.
That's how I got all my apps and purchased music on my wife's iPod Touch. I just transfered everything to her PC and then authorized my account on there with hers. Sync and done.