You can do what you want, but the result won't be what you want. They make hybrid drives to speed things up, but you need a special drive controller to work that way.
What you want to do is called JBOD (just a bunch of disks), combining different disks into one large volume. I do not think you can set that up with a boot drive. But as I said, it won't have the effect you want, will just add some seamless capacity without any speed increase.