Note: I have not been able to get working
I was researching this a little a couple of weeks ago. There were some posts on other sites that I came across that mentioned you can combine songs of two singstar disc, etc.
It said that you install one to your hard drive, then you copy the PKD files from another one into the USRDIR. So, I copied one SS to my external hard drive, then I copied the PKD files from another SS to that USRDIR. I had the original Pack0.pkd and Pack1.pkd. I copied the new PKD from the second SS title as files pack2.pkd and pack3.pkd. Once I booted up, the game worked, but it only contained one of the SS files and my downloaded content.
Downloaded songs are located:
The files end ".PKG.DRM". I think this means they are PSN signed package files.
I hope somebody with more knowledge discovers a way to download from SS site or rip protection and account info from purchased ones. This maybe possible with all of the tools that have been released, but I have not had time to delve into that.
Also, it would be great to be able to combine all the content from all discs including the songs from a SS for PS2 disc. The PS2 discs seem to have a different format. There has been some ramblings of people getting those to run from the hard drive too, but it was more complicated.
Hopefully somebody will figure it all out. Anybody else have info to share or have this working?