Hierarchical Db Extension

The US Dept of Veterans Affairs has primarily a hierarchical DB. I would like to extend Yii (and Gii) to do all the wonderful MVC stuff it does with SQL DBs to that DB. Which brings me to PDO. I don’t seem to be able to ascertain if it is really, really tied to SQL sources or not. Should I extend PDO or simply emulate it? And where do I tie in with Yii? I will certainly have to use things like CDataProvider, probably CArrayDataProvider, to be able to hook up with CGridView. How much further down the stack do I need to go? Any thoughts are welcome.