adding light to 3 way switches
The only way to do this would be to go in the attic. The basement 3-way has the neutral you need and the 3-way at the top of the stairs has the hot connection to the light. You need both wires to make your additional light work and those are only available at the existing light fixture. There's different types of steel raceway that can be surface mounted on the ceiling but doing that on stairs would be more dangerous than going into the attic. Maybe it's time to train a young apprentice.