I'm not sure what you mean by "semi-dormant ", but petunias are true tender perennials and can be overwintered quite easily. I recommend that you trim the plants back before bringing them inside and keep the basement quite cool.

You might prepare the trimmings as cuttings and try to root them in a porous potting medium. This is not the ideal time of year for rooting cuttings, but petunias are pretty cooperative. Eash stem should be prepared properly to insure success.

I wouldn't bother keeping the lights on for the petunias - buying new plants or seed will cost you less than the cost of electricity. If you're running the lights for other reasons/plants, it wouldn't hurt to try, but I don't think it's very smart in terms of $$$ to do it just for the petunias.

I agree, mxk3, thank you. And rhizo, thank you.