Can I store content in a relational database?

Yes, in several different ways, depending on your needs.

If you lots and lots of data, especially in legacy formats, you can use ZSQLMethods, a way to have Zope call SQL statements and you can then script the results. The data in the relational DB isn't really "content"; it can't undergo workflow or such, but for many casees, this is fine or even preferable.

You can use Archetypes with SQLStorage. This lets you work with normal Plone content, and choose which attributes you want to keep in the relational database. It's easy to use, but requires that you create custom content types--you can't use the normal Plone News Item, for example, and keep the data in the relational database. There's a long HOWTO on this at http://plone.org/documentation/archetypes/ArchetypesDeveloperGuide.

You can use APE, an add-on product for Zope that stores parts or all of the ZODB in a relational database.