Power a ceiling fan motor and light from the supply side of a three way switch
You could replace the 14/3wG between the two three-way switches with a 14/4wG cable, and use the fourth conductor to carry the switched-hot back to the 14/2 supply box, where you would connect it to the fan light wire. 

This is more labor than your other plan, to connect the fan light directly to the existing light, but it will be a lot less confusing to any future electrician or handyman who has to deal with the circuit.