The answer is, maybe.
You can do this, but there are several complicatied things to consider. First is that the speakers, hooked up in parallel (both wires running to the amp connectors) shows the amp 1/2 the impedance of one speaker (literally (Ia+Ib)/2). The amp may not like this. It may stress the amp an/or sound like dogs***.
Then there is the question of speaker efficiency. Inevitably one speaker will be louder than the other, and there's no way to adjust it.
Then there is the question of timbre matching. They just may not sound good together.
Overall you're better off not trying to use both.
Here's what you do: replace the front (LF/RF) Yamaha speakers with the JBLs. Calibrate the level. See how that sounds. Pay attention to how the JBLs get along with the Yamaha center. If you like it, keep it. If not, go back to the Yamahas.