The cut-off date typically suggested for seeding/overseeding lawns in this area is mid-October or 10/15. That doesn't mean it can't be done later, only that germination is reduced - sometimes substantially - because of cooler soil and air temperatures. Plus we are entering our rainy season and that can also impede successful germination. But you are right on the cusp so it won't hurt to try......all you will be out is the cost of the seed and some effort.

Overseeding really doesn't require much in the way of soil prep but it always helps if the existing lawn is in the best condition you can manage. That means as weed-free as possible, moss removed, recently aerated and dethatched. And mowed :-) Overseeding is not a one-step process and typically requires a couple of weekends for preparation. Warm season grasses are not typically used in this area - lawns here tend to be mixtures of cool season grasses. Most garden centers (avoid the box stores - they tend not to be very regionally-specific in their offerings) will sell blends formulated specifically for this region and an overseeding mix is commonly available although you might want to consider a mix for shady areas - limited sun is not very conducive to growning a healthy lawn, unfortunately.

After overseeding, should you attempt it this late, cover with a thin layer (~1/2" or less) of screened compost or soil. This will protect the seeds from birds and help to keep them in place when windy. The usual recommendation is to keep evenly moist until fully germinated but I doubt that's much of an issue now.........:-)

Should you decide to hold off until spring, fertilize now with a fall/winter lawn fert, preferrably organic. Since we do have cool season lawns, the fall/winter fertilizing is one of the best things you can do to energize your existing lawn now and encourage strong root development as it enters winter. Then start your overseeding prep in spring (April) with any necessary liming, thatching and aerating before seeding.