Hello and welcome. I don't think anyone has tried to install a center speaker yet. The base radio does not have a take off for it or for the optional amp on the infinity set up either. You may have to opt for an after market radio that will let you do this. In any case, we would appreciate any information that you come across on this task as most anything tried on a 2014 is going to be new information at this point.
I have only had the plate out once. I believe the vents pull up and out using friction clips. Then the plate unscrews pretty easily. I think Shaggy is right though. Adding a speaker in wouldn't be as simple as it seems. For me I think I will wait a while until the current sound system wears out and needs replacement.