Yes it's possible but depends what type of HDD you've got.
If you're using a 3.5" HDD in your A1200 already then you can just plug it into a normal IDE channel and access it using WinUAE. Google for a guide on how-to do this but it isn't too difficult. Just don't activate the drive in windows because it'll fark up the existing setup.
If you've got a 2.5" HDD such as one from a laptop then you'll need a 3.5" to 2.5" adapter from ebay like
this one... looks a bit crap but then mine is similar, anyway thats the "kinda" thing you'll be wanting. It just allows you to plug the 2.5" drive into the £.5" connector + molex power plug. Once again you access it via WinUAE without activating it in windows.
Andy