Is there any ready to use API provided by any eclipse plugin that allows one to manipulate UML class diagrams & database schema diagrams so that one can reuse it and for advanced users extend it to edit and generate same type or other non UML class diagrams / non database schema diagrams?
Not that I am aware of. You would need to use some combination of
EMF (Eclipse Mdoelling Framework) and
GEF(Graphical Editing Framework).