If I understand this correctly, you want to have a picture in picture of a live video from a camera shown using a Keynote presentation.
This can't be done from within Keynote, but a workaround is to connect a video camera, webcam, built in iSight camera, iPhone or iPad to the Mac, and swap between Keynote and Quicktime using command and tab keys on the keyboard.
- Connect your camera to the Mac.
- Open the QuickTime Player app on your Mac.
- Choose File > New Movie Recording.
- Click the Options pop-up menu, then choose any of the following recording options:
- Camera: Choose your connected iPhone, iPad or iPod touch.
- Microphone: Choose a microphone (if you have more than one available).
- Quality: Choose the recording quality. Maximum-quality recordings produce uncompressed files, which can use large amounts of storage space.