You want to add a second center channel *behind* where you are seated(?)
The problem with that is it will no longer be a center channel, but (center) rear. This will sound very odd, in addition to mucking up the surround sound field.
Having a photo of the space would be helpful in this scenario. You could upload to free to the following site, them post a link here to the URL. http://imgur.com/
Is there any space just below the HDTV, on top of the cabinet(?) Ideally, you would be disconnecting the existing (recessed) center channel and placing another one slightly forward, at least at the same distance from the back wall as the LG HDTV.
I have Marantz SR7010 receiver. I currently have a 7.2 set-up. The system provides the sound for a 65" LG OLED 4K TV. The TV cut-out area is is 82" x 82" with a 22" cabinet below which holds all the AV equipment. The TV is on an articulating wall mount so it is 22" out from the wall. The center channel and left and right front channels are on the wall behind the TV. The length of the room from the TV to the back wall is 13'. The width is 14'. I would like to add a second center channel because it is hard to hear. Raising the volume doesn't help, due to one open wall which provides an echo through my home. Is there a way to add a second center channel on the wall behind the sound field (behind sofa)?