If you really want to use your existing cameras, you can record the footage and compositive the video in a format that iPi DMC understands. Some users have done this with excellent results (1k0's
Danse Kabyle is a great example. Be sure to look at his 'making of' video, which shows him using two non-USB cameras along with a single PS3 Eye.)
That said, (and this is just my personal opinion,) it's far easier to use the relatively cheap ($30) PS3 Eye cameras and let Recorder all the work for you. The PS3 Eye cameras are nice because their 60fps frame rate reduces motion blur significantly and they are fairly equal in image quality right out of the box (i.e., no tweaking required.) And with the combined resolution of six cameras, the capture space can be quite large too.
G.