Normally, when you attempt to import footage from a different camera type than what's used in your current iMovie project (iSight footage is the exception—you can import it into any project format), iMovie won't let you and may give you the following warning message:
Don't worry; it can be done. If you need to mix footage from different camera types, follow these steps:
iMovie HD will automatically convert the footage so that it can be used in this project. Please be aware that depending on the project and footage formats, the footage may be converted to a lower quality.
If your footage is of a different aspect ratio, it may be letterboxed or pillarboxed to fit the project you are pasting into. If you need to add an effect to such footage, consider adding the effect in the clip's original project before cutting and pasting it into the new project.