Can you explain what you mean by "access that like an optical drive"? Because generally accessing an optical drive is pretty undesirable. They are slow and in a portable they eat up your battery life. You can certainly create disc images on the HDD, but there's very little I can think of that you would want as disc images. You just want the contents. Just put the contents on the external drive. Done. If you mean DVD movies, it's a bit more involved and requires extra software to do the ripping but still easily doable. You can get a several movies onto the free space of the internal drive, even, and many more onto an external of that capacity.

I may be missing something, but a DVD movie is about the only thing I can think of that we might still routinely play directly from the optical disc. This can most certainly be solved. Much better to have a few movies on the internal drive or a couple dozen on the external than to have a stack of DVDs with you.