OS X: Set up one folder as "extension" of another, symlink-style?
As far as I know, you can't do that directly but I'm not sure I understand why you'd want to?

Why not just symlink the entire folder and never worry about it going forward?

If it's important to you that it's only large files that are moved to the HDD, you could create a Folder Action in Automator and run a bash script that checked file size and based on that moved and created the symlink automatically (I'm assuming you've got some tech skill since you know what symlink even is :) With that said, I'd be happy to post more detailed instructions if need be).