I'm using MOSS 2007. All the document library created with default. In order to make search more easier, 5 required fields(as part of metadata) need to added to all the document library. Please advise on how to automate it for all.

You could try setting up the site columns as needed, add these to a dummy document library that is set up as you require it and then save that as a document library template via the Document Library Settings. Make sure you give it a recognisable name and it will be available for selection upon the new lists and libraries screen. Hope that helpshttp://www.final-exodus.net

You will not be able to edit the Default Document Content Type directly, but you can either create a new content type with the required columns and add it to all the document libraries or programmaticly inject those custom ones to the already configured document libraries