Hi, and welcome to the forum! I have a bit of a language barrier when trying to interpret the manual, but it looks to me like the MCD605 has speaker-level connections (the "+" and "-" connections) for the speakers and left/right RCA audio outputs.
I think the best and easiest way to upgrade is to get a subwoofer with speaker-level connections. Below is an image of what that would look like (and the speaker-level connections are labeled "HIGH LEVEL INPUTS FROM AMPLIFIER" in this example):
There are several subwoofers available with that type of connection. You would connect the speaker wires from the MCD605 main unit to the subwoofer's inputs, and then connect the MCD605 speakers to the subwoofer's outputs.
I recommend this so that you aren't playing the same frequencies through both the subwoofer and your speakers. With this type of connection, the speakers wouldn't be playing the same frequencies as the subwoofer because the subwoofer would filter those out of the signal that gets sent to the speakers.