Merging two databases to bring together all layouts, scripts, valuelists and relationships into a single file is not a simple task to take on and requires a good working knowledge of all these parts of the database file. Some aspects, such as the relationships between tables, cannot be imported or copied/pasted. They must be recreated by hand after importing the tables.
Other details, such as a layout can be recreated by copy and pasting all the layout objects from the original file into a new blank layout of the "merged" file, but careful attention must be paid to a number of technical details or you will get a great many broken objects on your layout that must be individually repaired before they will work.
See this thread on a step by step process for copy/pasting a layout: http://forums.filemaker.com/posts/a6d0528bcb/comments/201309/edit#201309
Scripts may be imported, but they may also need additional repair after import.
Repeated database design reports (requires FileMaker Advanced) made throughout the "merge" can be very helpful by doing text seraches for words like "missing" and "unkown" to track down loose ends that need fixing.
You may find it easier to simply edit the product database to add the contacts features you want, acquire a third party produced template that already combines them, or investigate a product called FMMigrator to see if it will make the merge process any easier. (I have not used FMMigrator so this is not a product endorsement.)
Thanks, PhilModJunk. I'll give these options a whirl.
Seedcode has a free calendar and provides instructions for integrating with your database.
If you have FM Advanced you can import tables, just FMPro you must create the tables field by field.
The instructions are here http://www.seedcode.com/pmwiki/pmwiki.php?n=SeedCodeCalendarFree.Integration
It is useful as an overview of the process.