You could use some of that RAM to create a RAM disk. There are commands that you can run from terminal or get some 3rd party GUI tool that allows you to do the same.
However, there's a huge difference between SSDs and RAM disk. RAM is volatile, SSDs are not. If your computer suddenly loses power, crashes, hangs or reboots, everything that was in RAM and on your RAM disk will be lost. Obviously the same will not happen with SSDs.
Having said that, you might not realize it, but OSX already does something similar to what you are thinking of doing. All that free memory in your computer is not being wasted. OSX automatically uses it as a cache. Whatever your computer reads, gets cached in free RAM. The next time you need that file, it'll be available to you nearly instantly.
Take a look at this answer, it's much more eloquent. Isn't Inactive memory a waste of resources?
1What possible benefit would you hope to obtain this way?! Assuming you could pull it off, you'd be trading RAM, the most valuable resource for avoiding massive amounts of I/O entirely, for Fusion Drive, an awkward compromise that makes some I/O a bit faster. – David Schwartz – 2013-04-04T23:18:06.660
Also RAM is volitile memory. Once the power is cut off, all of the data that was on it is gone. – cutrightjm – 2013-04-05T00:09:44.373