Hi JM, it depends, you can use the update feature that will work, but this will only work if the base database properties are the same, for example you are trying to merge a tgp database with a tgp database.
Here is a video that includes the procedure to do this:
http://www.pieslices.com/dreamsubmit...g_Updating.wmv
Remember, you can use the update or import button, but if you use the import button it will also import duplicates, if you use the update button it will not import duplicates, but it will overwrite any of the same sites. You can get away from overwriting duplicates by locking all the sites by highlighting them all in the database, right clicking and selecting lock selected url.
Hope this helps!