?
"Mikepier" wrote
My question is can I tie in the HW heater duct to the existing 6"
furnace duct, possibly using a Tee of some sort?

No, that is a code violation and may be too much for the 6" to handle.

I'd go with what RBM suggested. Check out what kind of vent is needed on
the new one also. Rheem makes a power vent model that just needs a PVC
pipe. I have no idea of the cost or if they are as trouble free as most
water heaters are.