The only way to get constant power to the lower box is to disable the switch in the first box and use the 3 wire cable to pass constant power and the neutral (which should already be there) to the second box. This will cause the first switch to be unusable, although the existing fixture could still be operated from the bottom switch alone. If this is acceptable, simply remove the first switch, splice the black wire to the black wire, white wire to the white wire and cap off the red wire by itself with a wire nut. Use a blank cover plate to replace the switch plate.
Otherwise a new source of power will have to be added to the second box. While this is possible, it is probably easier to use an "old work" box and add a completely new box nearby the old one.
I assume you are adding a third switch, to be located in or near to the lower box. If the new fixture could be operated from the same pair of 3 way switches either the lower switch or the light fixture has the already switched power available without the addition of a new switch.