Does implementing the Zend Framework require the existing MySQL database to be modified?
If so, what would the modifications be?
It would also be great if some useful resources for migration to Zend could be suggested.
Zend framework is not bound to a database. Hell, it can even exist without any database at all. Or different databases. Whatever you want.
You just need to connect to the database correct, and do the correct mappings.
If your database is in a sensibly normalized form, you should not have to make any alterations in order to implement Zend models.
I would recommend however trying a fresh project from scratch using the Zend tutorial here: http://framework.zend.com/manual/en/learning.quickstart.create-project.html
No, there's absolutely no need to change your database.