Is there a way to insulate a 3 season tent to make it more practical for winter camping? I have heard that putting a wool blanket between the rain fly and the tent proper works. ( I assume that means on top of the tent poles not directly on the tent material itself ) That seems like it would work to help keep the tent warmer. (no other source of heat but body heat) What about using mylar or some synthetic blanket material? Condensation a problem? Looking to possibly extend my camping season but not interested in going all in for winter camping.

I would not spend a lot of effort to insulate a tent- unless you insulate all the walls and the floor. I believe to it is best to have a great sleeping bag set up.
Condensation is always an issue. If it is waterproof for sure it will be an issue. If the temperature is at or below condensing within the material it will even condense in a breathable material. Sleeping bags will condense moisture vapor at the point where in the insulation it is below the freezing point, without regard to the material. A vapor barrier close to the skin will keep you sleeping bag dry. I often will "dry" (sublimation) my sleeping bag when cold camping, as I do not use a vapor barrier.

Seems like an insulated tent would be a moisture, condensation, and ventilation nightmare. My winter tent is only warmer when the stove is running.

I think you would be best off to make sure your sleep system is adequate. And fiqure out a small stove for your existing tent.

Echoing what others have said....

I think insulating a tent in the winter wouldn't improve things near enough to justify the extra weight/space unless you're somehow adding a stove too.

Barring extreme blizzard conditions, sleet and or rain; a good sleep system will keep you 'toasty' even without a shelter.