Can I make a DIY Fusion Drive?
Yes, you can create a fusion drive on older Macs, described in full length here:


You obviously need both an SSD and a HD. Let's assume they are known to the OS as disk1 and disk7. You can check disk id by diskutil list.
diskutil cs create bla disk1 disk7 to create it, where bla is the name you give to the fusion drive.
diskutil cs list and take note of the Logical Volume Group ID
diskutil cs createVolume &lt;ID from above&gt; jhfs+ blub 100%, where blub is the name you give to the volume.


Link to Apple instructions: Click Here 

Note: These are instructions to fix a split fusion drive, but work also for making one.