Is there a simple way to generate an ERD in Visio from an existing MySQL Database? I'm aware that this is possible in MySQL Workbench, but I have a requirement for it to be done in Visio. Or otherwise can I convert an ERD made in MySQL Workbench to a format Visio can open?
Yes, it can be done in Visio. You need to use the reverse engineer feature.
Here is a blog post explaining the basic steps:
http://maniish.wordpress.com/2007/11/24/reverse-engineer-mysql-in-visio-2007/