You can't, simply. An iPhoto Slideshow can only draw from a single library, so you'll need to get all the images into one.
Alternatives to iPhoto's slideshow include:
iMovie is on every Mac sold.
Others, in order of price:
PhotoToMovie $49.95
PulpMotion $129
FotoMagico $99
Final Cut Pro X $299
It's difficult to compare these apps. They have differences in capability - some are driven off templates. some aren't. Some have a wider variety of transitions. Others will have excellent audio controls. It's worth checking them out to see what meets your needs. However, there is no doubt that Final Cut Pro X is the most capable app of them all. You get what you pay for.