no problem doing this. The plug is the same thread as the other ports.
teflon Tape all connections... and put shutoffs with check valves to make sure the air pressure of the kegs are isolated. (otherwise a leak in one, or a pull off of the other kegs... leaving 5 flat kegs.
I put 6 into my fridge, and put two other ports (one for my beer gun, and one for a keg outside of the fridge for times when I needed to pressurize before storing)
The other option is to get another two port or more maifold and connect it to the plug port.
Just make sure if you have taps out the front of the fridge, you look to see if you have spaceing for it. I put 6 kegs into my fridge, bought 6 taps... and just did not have a wide enough front on the fridge to make it look good. (So I only have 5 on tap through the door)