Is there any way to add one to the C4500 (6.6 allison) (it currently does not have one)
Is it as simple as plug and play or is there some major work that needs to be done?

It was offered as an option so you should be able to plug and play. When I had my 4500 I inquired about the same thing and they said it would hook right up.

Mine has it and doesn't work, lol! Its never moved off the 160 deg mark and I know the tranny heats up pulling the trailer.

I installed the Edge Juice with ***. and I use that to help monitor the trans temp as well as the engine temp and of course the EGT. The 04 4500 and the Edge work well together.

I have the temp gauge from the factory too. I didn't think it worked until I was pulling 20k up hill. The Allison is built for this kind of work and does not get hot. EVER. Unless there is a problem, you will probably never even see the needle move far from the pin. It isn't built like the idiot gauges in the dash where "normal operating range" always has the needle pointing straight up.

X2

I have the temp gauge from the factory too. I didn't think it worked until I was pulling 20k up hill. The Allison is built for this kind of work and does not get hot. EVER. Unless there is a problem, you will probably never even see the needle move far from the pin. It isn't built like the idiot gauges in the dash where "normal operating range" always has the needle pointing straight up.
Good to know, I'm still learning more about that tranny as I'm new to the GM truck world having run my Dodge for 250k+ miles and still going. I just remember that tranny running around the 180 temp range when pulling my trailer. I figured it was either the gauge was broken or it has a massive fluid reservoir which allows it to stay it cool.