Either add a SATA controller card that will allow adding more drives than the motherboard supports through its available SATA ports, or add the drive to a USB 2.0 or 3.0 connection. If you want to span the drive between more than 1 drive you can use a symbolic link, however I would be very careful with going the symbolic link route if your looking to combine drives for combined capacity as for if say the drive with a symbolic link over USB was disconnected and you were in the middle of running a game where files for that game were on the external hard drive connected via USB 3.0 your game would crash and could corrupt the game depending on what was happening when the external drive was "accidentally" disconnected when in use.
Adding a SATA controller to a available PCIE slot is the best method, best performance, and less likely for data loss. The USB method is risky if the connection is interrupted. Symbolic links if used have risk of corruption of data if drive is improperly disconnected while in use.
