Schema overlay question

Hi All,

A simple question to the list.
We are designing a database, the database has three major components (each component with 4-8 tables)
two components are ready, is it possible to 'overlay' the third one later? or
Do i have to drop all the components (along with data!!!) and then add the third component and then build the entire DB?
What i am saying is some tables are existing, could i later add a bunch of tables at a later date without dropping the entire database?
Thanks for your help.

Regards,
Hrisih