There are a few brands & configurations (single-channel supports two drives max., dual-channel supports four) of third-party PCI IDE controllers available now for not a lot of cash. You might even find one available on an on-line auction service if you do a search.
The 8500-series boxes will accept & work fine with any of them; just make sure your power supply has an available 4-conductor power cable available to use with the drive (or drives) you want to add. Some cards come with cable adapters that allow use of two drives on one PS plug.
Physically mounting the drives might be your biggest problem; I've never had an 8500 to look into; 9500's required a plastic drive adapter thingy & only had two (I believe) drive bays that could accept 3.5" form-factor hard drive devices.