Is this possible? I would like to be able to play regular CDs on my players (Denon DN-S1000s) while hooked up to SSL. Is this possible?

Yes, when the "thru" outputs of the hardware interface are connected to the line inputs on the mixer your CD players will play thru the hardware interface and into your mixer :-)

I normally hook my CD players to Inputs 1 & 2 on the SSL interface and hook up the Line Outputs to my mixer inputs. So, the one change I need to make is change the RCA from my mixer inputs to the Thru Outputs from the interface. Am I correct?

You'll need to have 4 line inputs connected to play your music from Scratch LINE and directly from your CD players without unplugging anything. 1 pair for the line outs of the interface and the other pair for the thru outputs of the interface.