Hello there, ssegy10.
The following link to the online iMovie Help menu provides the information on how to accomplish what you're asking:
Work with multiple libraries
http://help.apple.com/imovie/mac/10.0/#mov3fa25bae7
- Connect a storage device that contains the target library to your Mac, or copy the target library to your Mac.
- Choose File > Open Library, and choose an option from the submenu.You can choose from recently opened libraries, locate an existing library on your Mac, or create a new library.The selected library is opened in the Libraries list, with the first event selected and its contents displayed in the browser.
- In the Libraries list, select the event that contains the item you want to move or copy.
- In the browser, select the item you want to move or copy.Tip: To select multiple clips or projects in the same library, hold down the Command key as you click the items you want to select, or drag a selection rectangle around the items.
- Do one of the following:
- To move items between events or libraries: Drag the clip or project to another event or library.
- To copy items between events or libraries: Option-drag the clip or project to another event or library by first starting to drag and then holding down the Option key as you drag.
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.