I am replacing a 30 yr old oil furnace with wood add on. I have two existing steel chimneys being used for both units. These flues measure inside as 6 inches. The wett thech was in today and said new furnace openings come in 8 inches and you can only adapt fr 7 to 6 inches or 8 to 7. He stated that my chimneys will probably have to be replaced. I have an older house which is two and one half storey high, lots of pipe. My question is are there any oil furnaces with wood add ons or wood oil combos that would accommodate my existing chimney?

I doubt it

A 6 inch flue is kind of small for a combo unit. You could shop around to see if you could find something that would work BUT the smaller units output (BTU's) will probably be less also. Flue size is usually determined by the BTU capacity of the furnace.